To all Cambodia experts, April next year, I will be visiting Siem Reap for 6 days. Any advice apart from the Angkor Wat temple tours, what things or sights to do? I am not planning to do PP this time because of the limited time that I have. Will this be a mistake?

It would be great if you could stretch it to 7 days, then you could have 4 days in Siem Reap, 2 days in Phnom Penh and most of one day travelling by bus between the two. I think Phnom Penh is well worth it. You can fly between the two instead of the bus trip, which might give you an extra half a day. Scroll down through the Cambodia branch, as there have been several other Siem Reap questions recently.

@dale164 - the tickets already booked because of some promo... i cannot extend it now... is PP similar to Bangkok or Saigon as I have been to these 2 cities??
PP is a different city with a very different feel to that of Bangkok. I had some friends who went over to Siem Reap last week from Thailand and were amazed as how much different it is.
Below is a slightly edited version of what I sent to them.
An obvious start would be Angkor wat and surrounds. You can buy a one day ticket for $20 or a 3 day for $40 (ticket also gets you into some other temples nearby). If you are going to get photos it might be worth spending more than one day there, you will do a short tour and a long tour. Find a tuk tuk driver he will know where to go. about $15 a day, more if going to far off places. For Beng Maelea and surrounds it cost us $30. I would also take a ride in the static balloon tethered near Angkor, it used to be $15 for about 15 mins but it puts the whole area in context.
If you have time certainly take a trip out to Banteay Srei and Beng Maelae it used to get no visitors (think its changing though). Phnom Kulen and Kbal Spean are also easily accessible now.
Certainly take a trip to a floating village. I would choose Kompong Pluk over the others due to its towering houses on stilts and flooded forest, take a boat around, prices can change and access was changing when I was last there, estimate $40. The village of Chong Kneas is packed with visitors and its detracts from the village itself.
Take a trip down the side of the river to the tonle sap, either on backs of motos or in a tuk tuk (foreigners can not rent motos in Siem Reap anymore). Motodops are a popular way to get around and are normally a dollar a time within the city. I think a tuk tuk is now $1.50-2 across town. Its a cool trip with a small hill to climb at the end, but the highlight is the market near where you catch the lake boats, if you get there early there is all sorts of shit going on, worth seeing in my opinion.
In the city there are a few things to do. If you've not had enough of Angkor temples, then the Angkor National Museum is worth a visit, its well curated and lots of info on display - located north of the city beyond the royal gardens. At dusk in the royal gardens there is a display of fruit bats. The walk along the river bank from the old market area up to the royal gardens is a nice walk. There is the Cambodian Cultural Village on route 6 going east, its a bit cheesy and only go if you're bored, ironically the Khmers love it. Then there is the landmine museum, also in the same area.
If you want a quiet afternoon around a pool with cold beer and western food not packed with tourists then I would recommend Aqua its owned (or at least was) by and English guy John, I've not been for 3 years so not sure what its current situation is. If your feeling flush then there is always the FCC by the river.
Eating, Cambodian street food isn't in the same league as Thai food, it can take some getting used to. However at the end of pub st there is always a night market with food stalls, its cheap and tasty, even my parents happily ate there. Alternatively there are the bars along pub street. Le Tigre De Papier does great food and awesome pizzas. You have to at least try the Temple Bar (much cleaned up since I first entered) and Angkor What? bar is popular and plays decent music. Warehouse has live music and is more of an expat hangout. Down the Alley next to pub street situated between there and the market you'll find a host of great eating places. BTW beer is dirt cheap. X bar can be pretty cool, its at the east end of pub street situated on a roof, it has a skate ramp too, open very late with good live music.
The old market (Psar Chaa) is well worth looking around although the smells can be a bit full on if you're not used to it.
I would spend at least on evening in a Khmer Beer garden and night club just for the experience. Maybe try Hiphop club. Its very different. At a beer garden you just keep ordering cheap beer and food. The best area is just north of the route 6 a block back. just take a wander around, ask a tuk tuk to take you.
Its a pretty safe place, just beware of bag snatchers taking stuff from your tuk tuk and kids stealing flip flops from your feet then charging you to get them back. The operate near Molly Malones Irish bar and outside the minimart under the X Bar.
If you happen to want to leave Siem Reap, I would strongly recommend the boat trip to Battambang, its one of the best boat trips I've done in Asia. Beautiful and it takes you on the Tonle Sap and up river to Battambang, great to see life along the river. Battambang itself is a chilled small city. You can get a bus back to SR a day or two later.
Alternatively head out for an overnight trip to Kampung Thom and take a trip to Sambor pre Kuk temples.
Visa on arrival is very easy just take a couple of passport photos with you and preferably a $20 note. ATMs dispense $$$, you'll get Riel back in small change. If you need to change money go to the gold shops at the markets they offer the best rates. You can't change Riel outside Cambodia, if you happen to be left with any small change be sure to change it into dollars.

It will be strange feeling if you try yourself with biking tour in Siem Reap, Camdobia.
You can spend 5days sightseeing and cycling in Angkor Wat.
There is possibly no better way of discovering the ancient Angkor temples than cycling along forest tracks and emerging upon them. This 5 day cycle tour allows you to fully explore the Angkor temples at a relaxed pace as well as enjoying a delightful cruise on the Tonle Sap Lake (the largest in South East Asia) and witnessing its fascinating floating villages. Plus in your free time you may wish to take advantage of the many excellent spas in Siem Reap and receive a soothing massage, your reward after a few days of cycling. I know a tour like that on site Bikingvietnam.com of Mr. Joe Nguyen. Schedule as follow:
D 1: Arrive Siem Reap, the home of Angkor Wat
D 2: Exploring the Magnificent Angkor Temples by Bike
D 3: Cycle Roluos and Cruise the Tonle Sapa
D 4: Cycle Banteay Srei & the Grand Circuit
D 5: Free until Departure
You can check detail in that site. Have a nice trip, guy! :)
great advice given above.
i'm a history and temple buff, and the temples around siem reap are totally amazing. i have been there 3 times, and i'm already planning my next trip there. the most famous temples (angkor wat, bayon, ta phrom, banteay srei) are beautiful and not to be missed. as you will have 6 days around siem reap, i would spend 4 days visiting the temples. hire a taxi, a tuk-tuk or a motorcyle (all of them with a driver of course), or rent a bycicle to ride around yourself. when visiting the "angkor park", you can do a very good mixture of visiting very famous and much visited places, and also visiting less visited temples. they might be not that spectacular, but will be much less visited, and personally i have had some of my best experiences in some of those places. a highlight was beng mealea, which is about 50km from siem reap.
other things to do / experience in siem reap ? food is definitely a highlight for me. plenty of places to have great food. you might also want to visit a floating village on tonle sap. maybe have a massage, or have a "fish spa".

@all - thanks for the advice... really appreciate them.... also looking forward to this trip very much....
Very good advice but a few update:
If you have time certainly take a trip out to Banteay Srei and Beng Maelae it used to get no visitors (think its changing though). Phnom Kulen and Kbal Spean are also easily accessible now.
Both get very busy now, go a bit further to Koh Ker or even Prea Vihear.
I would choose Kompong Pluk over the others due to its towering houses on stilts and flooded forest, take a boat around, prices can change and access was changing when I was last there, estimate $40. The village of Chong Kneas is packed with visitors and its detracts from the village itself.
Agreed. Again, there is an even better village heading the other way.
Take a trip down the side of the river to the tonle sap, either on backs of motos or in a tuk tuk (foreigners can not rent motos in Siem Reap anymore). Motodops are a popular way to get around and are normally a dollar a time within the city. I think a tuk tuk is now $1.50-2 across town. Its a cool trip with a small hill to climb at the end, but the highlight is the market near where you catch the lake boats, if you get there early there is all sorts of shit going on, worth seeing in my opinion.
There are now electric bikes for rent, you do not really want to go cycling in April. These days you need a ticket for the hill (Phnom Krom).
Then there is the landmine museum, also in the same area.
That is the war museum, the landmine museum is inside Angkor Wat.
However at the end of pub st there is always a night market with food stalls, its cheap and tasty, even my parents happily ate there.
Gone, but there are still plenty of Khmer restaurants around. And Pub Street is generally the worst place for food though Le Tigre is indeed good (had lunch there today)/
Its a pretty safe place, just beware of bag snatchers taking stuff from your tuk tuk and kids stealing flip flops from your feet then charging you to get them back. The operate near Molly Malones Irish bar and outside the minimart under the X Bar.
And stay well clear of the women with the babies asking you to buy milk powder, it is a scam.
If you happen to want to leave Siem Reap, I would strongly recommend the boat trip to Battambang, its one of the best boat trips I've done in Asia. Beautiful and it takes you on the Tonle Sap and up river to Battambang, great to see life along the river. Battambang itself is a chilled small city. You can get a bus back to SR a day or two later.
Battambang is great but the boat might take very long in April due to lack of water.
Thanks for the updates and corrections Hanno, I know you are based there. I was out for some beers with some friends who work in SR just the other day and they told me the food stalls had gone, I just forgot to omit it from my post.
What is the name of the of the better village going the other way? Also is Aqua still worth recommending?