Boat-trips around the fortress.
Climbing the bell tower of the gothic Saint Vitus Church.
Bicycle Day Trip : Rent bike at railway station in Weesp. Cycle to Naarden along natural reserve Naardermeer, and visit other fortified towns Weesp and Muiden with medieval castle. Marked bicycle-route Boerenlandroute 25 miles, available at tourist office VVV. There's also this link-page about bicycle day trips in the region. Bike rental at train station Naarden-Bussum is another option.