You will have plenty of time. Take the train into Amsterdam Centraal Station. The trip takes maybe 15 minutes. Cheap too. The station is right in Schipol. Cabs take too long and are expense. As long as you leave Centraal 2.5 hours before your flight, you should be fine. Got to the Rijkmuseum. Plenty to see and the promenade outside is a wonderful place to grab lunch (if the weather is nice).