You can rent kayaks cheap at Gull Pond. Gull Pond connects to two other ponds, so you can have a nice time paddling around for a few hours and jump in the water any time you feel too warm. You can bring a picnic, too. Entirely pleasant despite my aversion to unsalted water. Had a chat with a fellow there from Maine. Complained to me that the water was too warm to be refreshing, but his kids seemed to love it.
No powerboats are allowed on any Cape "ponds." That's a welcome community policy. They are really small lakes.
Neat thing about Wellfleet: Lakes, harbor, ocean and bay within a mile's drive or bike. Lots of different ways to play in the water. What else is there to do in the summer, besides reading books and playing tennis? August is my personal Robert Parker Fest.
Next trip, I will bring camera, rent a kayak, and paddle around Nauset Marsh in Eastham at high tide and check out the wildlife. Gosh, I sure do love being up there in the piney woods and the cool salty air. Thankful, too, that both of my parents can still enjoy it. Yes, my Mom still kayaks and chows down on Littleneck clams and Wellfleet Oysters. Dad is a bit too rickety for kayaks but his brain is as Yankee-acerbic as ever.