Some people like to swim in the ponds on the Cape. I've never done so. We like to beach hop, but with several young children, you will probably want to be fairly close to one appealing beach.

The beaches have parking fees, but a pass is often good at more than one public beach in the same town. The Cape Cod National Seashore pass is good at their six beaches.

You may be able to get a weekly pass at a discount for the town beaches in the town you're renting in. The National Seashore also has daily and seasonal passes.

Skaket Beach in Orleans, which is very pretty, is calm. It would be nice for young kids. For a bigger beach with bigger waves, you can also go to Nauset in the same town. (Parking pass is good at both.)

There are bike rentals on the Cape. I don't think you'll have any problem finding a shop.