Worth while visiting the east of the country, Arnhem.
The beautiful Dutch Open Air museum, with old buildings, crafts, mills and a very modern exposition of the history of the NL with inter-active games. Worth a visit, possibly too much for one day.. (44 hectares)
Arnhem is also well-known because the WW II. The allied army lost
the battle of Arnhem in 1944. Airborne museum in Oosterbeek. The airborne field in the neighbourhood. Also the war museum in Groesbeek (near Nijmegen) is worth a visit. Every year their are celebrations, and this year it is 75 years ago, that the NL have been freed.
Not far from Arnhem there is the Kröller-Muller museum, with a lot of modern art (many paintings from Van Gogh) in a beautiful park, that you could cross by bike. (After entering this park the bikes you may use for free!)
Okay, you would like to go to Amsterdam? No problem, the train will bring you in one hour from Arnhem to Amsterdam.