Is the itinerary a bit too rushy?

If you're going to visit Giethoorn, i suggest you visit Dwarsgracht as well. It's not that far away and in my mind it's less 'touristy' and a bit more unspoilt than it's bigger brother Giethoorn. I agree with Arizona the best way is probably to rent a bicycle at Steenwijk train station. The surrounding are consists of the two connected nature conservation areas De Weerribben and De Wieden and are amongst the most beautifull in the Netherlands. The best parts though can only be reached by canoe or electric boat, both of which can be rented all around the area.

Rabblap, your suggesion sounds quite appealing to me but it is possible to visit both Giethoorn and Dwarsgracht in one day? We are quite pressed for time : (

#11,12: your description is to-the-point, rabblap.

IMO very doable in one day, from Steenwijk to Giethoorn/Dwarsgracht is only 8 km.

Signposted from the Steenwijk railwaystation, you follow 31-39-62-60-61, for the greater part along a canal. This is a socalled knooppuntenroute = special cycle route with numbered hubs. A map is useful, get one at the Tourist office, called VVV, in front of the station.
Train back home to your base in the evening. Have a break in Zwolle for a nice dinner in the historic towncentre. (from simple to Michelin ***) BTW you may interrupt your trainjourney in NL, not always possible in the world I learnt...

But decide on basis of weatherforcast. Skip the trip when weatheronline predicts > 25 % rain

Thanks a lot Rabblap and Arizona. I'll take your advice, hopefully the cycling itself won't be very tiring ^^

Steenwijk - Giethoorn is doable by bicycle, i've done it probably a few hundred times :)

But if you want to save energy, at Steenwijk train station you can rent electric assisted bicycles:
http://www.snorrenindekop.nl/index.php?parent=4&subpage=90

One more tip:

stop for a drink at Jonen, a small village that can only be reached by bicycle or boat. It has a small ferry for bicycles to cross the water.

http://maps.google.nl/maps?hl=nl&q=jonen&ie=UTF8&hq=&hnear=Jonen,<i>Steenwijkerland,</i>Overijssel&ll=52.719119,6.017225&spn=0.003925,0.01545&t=h&z=17

This plan is getting more and more exciting now. Thanks again Rabblap ^^

With such a short period of time allocated for The Netherlands, I would certainly skip Rotterdam, the ultimate ugliest thing in the whole country, and visit more of the country itself. You could even replace the day in Rotterdam by a day in Enkhuizen, in the north. It only takes 1 hour to reach by train, it's at the end of the line, 1 train per hour. Read about the fascinating history of that city. Zuiderzee Open Lucht Museum (Open Air museum) there is worth every single minute, you reach it by boat from behind the strain station.

About those many places within 1-hour reach from Amsterdam: here are several ones.

Congrats for your intention to go off-the beaten path, Giethoorn and all. Not so common to see a 1st time visitor interested in s/thing else than Amsterdam. You will leave sorry to have such little time for visiting it (it = Amsterdam and The Netherlands) - it may look tiny on a Europe map, but it certainly has a LOT to offer!

Ha ha yes I agree that Rotterdam is ugly. I stated it before as well. I have kind of a love / hate relationship with the city and lived there for a few years in the past.
But I also described its plus sides in reply #9.

I agree that the Zuiderzee Museum in Enkhuizen is great fun. They rebuilt a small village in old style, including shops, craftsmen (for example a sail maker) and a school and such. If you do go there, definitely try to enroll for the writing class in the school! You'll have to write using an old fashioned pen and ink from a jar, and the teacher will scold you if you spatter ink by writing too hard. :-)

I guess it's now up to the original poster: what are you looking for? There's enough to do to fill weeks.

I would certainly skip Rotterdam, the ultimate ugliest thing in the whole country
I assume you've never been to Hengelo, Tilburg, Lelystad, Almere or the Westland area ;-)

I wouldn't include Rotterdam in a whirlwind trip of the Netherlands (unless you're into big ports or want to see the Boymans van Beuningen museum), but in all fairness I think dismissing the whole city as "ugly" or even the "ultimate ugliest thing in the whole country" is a bit of an exaggeration. Rotterdam is definitely not your typical quaint Dutch town, and when it comes to historic architecture it can't hold a candle to Amsterdam, Haarlem, Leiden, Delft and 150 other towns, but it does have a lot of character, grittiness, some interesting modern architecture and a down-to-earth atmosphere that has grown on me over the years (I haven't lived there but visited quite regularly).