From Schipol, get the train to Amsterdam Zuid (one stop, 2.50 Euros)
Get a 5 tram to town, get off at Leidesplein. Total travel time 30 mins
Have fun in town for 2 hours
do the reverse - 5 tram back to Zuid, then train back to Schipol. Again, travel time 30 mins.
Avoid the hell that is Amsterdam Central station.