Two popular destinations for a trip like this are Haarlem and Den Haag. I've always been too busy in AMS, but people that have visited these cities have enjoyed them, especially Haarlem.

To get the train to Haarlem requires a change at Amsterdam CS, but the total travel time is only 32-38 minutes and EUR9.10/14.80 return.

There's a n/s train to Den Haag from Schiphol at 13:04 and every 30 minutes thereafter. It takes 29 minutes and is EUR12.80/20.90.

Delft is another option, a bit further. It requires a transfer at Den Haag.