A gentle warning:

As you walk the streets of old amstedam, particularly the areas around the redlight district, beware the men who come out after dark, and will stand on the corners of canals and streets. You will know them becuase when you walk past, you will immediately hear:
"coca? Extasy? Cocaaain?"

For the safety of both you and your wallet, ignore these people. Every time I've heard about people using these guys, they got ripped off one way or another, sometimes just loosing a bit of money, sometimes loosing all the moeny they had on them.
Even if you get refused entry to a coffeeshop because of your age or something, DO NOT USE THESE GUYS!!!

The Van Gough museum is worth a look when you have just left a coffee shop, although it can be a bit busy & intense.
Avoid Smullers fast food as well, its vile, but there is an excellent juice bar in the station.