If you park at the lake there is a small path that will wind around to the local public beach. It's not very big, but it's a good place to relax and hang out with your friends. Just remember to use sun screen. Found THAT out the hard way! (Higher altitude causes the sun to react quicker on your skin.)
Spend time at or on the lake. There are little beaches and coves everywhere to go swimming. If you have a boat, use it! The lake is huge, and you can spend lots of time exploring. It's also very canoe and kayak friendly. Places around will rent them - try Winnepesaukee Kayak on Back Bay or Brewster Academy.
If you want to get away from the heavy motor boat traffic on Winnepesaukee, go to Crescent Lake or Lake Wentworth - both about a mile or less from Wolfeboro. There is a loon sanctuary you can visit.
There are lots of parks nearby that are good for picnics, etc. Try Cate Park in downtown Wolfeboro or Clark Point at the end of Clark Road.