Get onto Lastminute.com they have some super deals. Try tracking down Galaxy hotel in Amsterdam, we stayed there 2 years back and it was well posh but sooooo cheap, and the foot ferry journey home after a night in town is an experience and a half!!!! Have a great time.
Me and my friends went to amsterdam last year and stayed on a house boat there was 5 of us - it was wicked try www.amsterdamhouse.com (search house boats in amsterdam).