Great question! I live on Cape Cod, so I'll help you here what I can.
Cape Cod things to do........
Beaches (Nation Seashore a must, Craigville, West Dennis)
Cape Cod Rail Trail (a bike path along the Cape) in Dennis you can start and even rent bikes.
Heritage Museum and Gardens in Sandwich
Plymouth Plantation (this is in Plymouth on the way to the cape)
Main St in Hyannis, Chatham, and P-Town (please be forwarned that P-Town is the mecca of gay people. We locals don't care, we know... and still have a great time there. But I have seen on this very site, some people who were upset they had no idea and weren't warned. Pilgrim monument and museum are in PTown too)
Scargo Tower in Dennis
Whale watches or seal watches (also available in Boston as well as the cape)
Ferry Day trip to Nantucket or Martha's Vineyard
Meal at Seafood Place (a must if you love seafood and are going to be on Cape)
Cape Cod Railroad
Fishing Trips on a Boat
I agree with the poster who said that you should come during the week. We locals don't go over the bridges unless we have too, and we avoid it at all costs during the summer. It can take you 2-3 hours to get on or off Cape on the weekends.
If I come up with anything else, I will come back. If you have any questions, just let me know! Have fun, and we'll be happy to welcome you to our Cape!