There are luggage lockers throughout Schiphol Airport.

The best place to sleep is near the meditation center and the KLM Crown Lounge (no. 41). Just head upstairs past the casino. There are some great designer chairs that resemble business class seats in a public area there.

To get to the city center, take a train directly from the station at Schiphol to Centraal Station. 15-20 minutes each way. €3,60 each way.

Everything you could want to know about the coffeeshops in Amsterdam can be found here.