Exploring Cambodia ideas

Spend a night in Kampong Thom or Pursat, or a couple of nights on the coast.

if you have the time, try an elephant trek in mondulkiri. it's a long days travel from PP but worth it.

I enjoyed both Sen Monorom and Bokor Hill station very much. Bokor Hill Station is not too far from Phnom Penh. Maybe you can consider it in your trip.

I was in SR for 9 days and every day I saw a different place, all of which totally worth the trip. It's an area that offers really a lot of options... too bad that 95% of the visitors stop by just for a couple of days.
Angkor is best seen at a leisurely pace, giving yourself enough time to assimilate the details and not confuse one temple with another. But there is not JUST Angkor. You can visit the floating villages (Kompong Phhluk is my favourite). You can climb Phnom Bok, Phnom Khrom, even go to the top of Phnom Kulen. Kbal Spean is also very interesting, 2. Banteay Srei and Banteay Samre should not be missed, and the outlying temples of Beng Mealea and Ko Ker are fascinating as well, with their touch of mystery and roughness.. You can visit the countryside on top of a motorbike (with a driver) and get lazy in the rural village of Rulous (getting the chance of visiting the Rulous Group, which is very interesting, especially if seen before Angkor). Get up early and beat the crowds. Rest on a hammock sipping some coconut juice in the middle of the day when everybody is crazily running around the sites. Go back out again when the air is less hot and the light beautifully touches the temples. In Angkor, the Bayon, Ta Phrom and Angkor Wat should be visited on different days, so that you can enjoy their beauty and still have some time to assimilate their details. You can visit Angkor Thom in one day and see the sunset from Phnom Bakeng. You can visit the area east of Angkor Thom in the following day. And you can end with a morning in the Eastern Baray area (plus Preah Khan) and spend the afternoon in Angkor Wat (or the opposite).
Whatever you decide, Siem Reap area is not a place to rush. Spend some time there, and you won't regret it.

The countryside around Battambang is stunning, and the ruins of Banteay Chhmar, up in the northwest are incredible.

A few days on the coast at Sihanoukville at the end of your trip will make Koh Chang easily accessible. Take the ferry from SV to the border at Koh Kong and then a bus to Trat/Koh Chang.

Thanks so much guys for your help. I'm sure I'll have a few more questions after a little more research.

GROOGER.... Any idea on travel time from SV to Ko Chang.

Cheers

The boat leaves at 1pm I think. Check that.
It takes 4 hours to Koh Kong and you will see some of the most unspoilt islands in the world along the way.
After that you'll have to get a mini bus to Trat (1 hour) and a bus to Koh Chang (1 hour or so).