Amsterdam is definitely walkable. In fact, in the several times I've been there I've never used public transportation, except for the train from the airport.

I loved just aimlessly wandering through the streets and along the canals. It's one of my favorite cities. Be sure to spend lots of time roaming around the Jordaan neighborhood -- lots of small shops, galleries, and cafes. Very homey and cozy, yet hip.

My day trip recommendation would be Delft. Very charming, lovely square, and worth exploring the side streets and canals. I liked it much more than Haarlem and Brussels, and I thought Zaanse Schans was a bit of a tourist trap. Have a great trip!