It's really not much time to do much of anything, except maybe the boat ride as metioned or just walking around a little bit. Are you new to Amsterdam? If so, the boat ride might be best to see the nicer parts of the city without worrying about getting lost. If you want to walk around, it's enough time to walk to Dam square, through the busy Culverstraat (shopping street) and then along canals towards the outerring and museum quarter. The problem is that should there be a problem, you risk missing your flight because there is no breathing room.

You could also head to Leiden, which is 15 minutes to the south and is smaller than Amsterdam so more managable in the time given, albeit not quite as nice. There are some museums and if the weather is good, canal-side cafes, restaurants and bars might be an option. There are also lots of cute little streets and alleys to explore...

A return to Amsterdam Centraal from Schiphol costs E6,40 and there are usually about 5-6 trains per hour that run directly (more if you count the slower trains). You can buy tickets in the baggage claim area to avoid possible queues for the machines closer to the station. Do not take a taxi into the city; it takes longer (to get to the center of town) and you will pay through the roof.