You can go to
Plimouth Plantation. It's kind of interesting, but really better if you have kids with you.
You can go to Plymouth and see the rock, throw coins at it, if one sticks that's supposed to be good luck.
🙂 You can take a look at the Mayflower II, again more interesting for the kids.
The National Marine Life Center is interesting.
There's the Pilgrim Monument and Museum in Provincetown.
Lots of whale watching tours available.
Lots of good biking. There's the Cape Cod Rail Trail, the Canal Bikeway, the Shining Sea Bikeway, and the Provincelands for something different. There a link to them
HERE
Martha's Vineyard and Nantucket are great for biking too. You can also rent mopeds on Martha's Vineyard if you want, I'm not sure about Nantucket. Lots of shopping on both islands. Moshup Beach and Gay Head on Martha's Vineyard are nice.
Lots of good antique shopping all over Cape Cod if you like that sort of thing. Not a lot of bargains though.
The Woods Hole Oceanographic Institute is interesting. You'll probably want to take the ferry from there to Martha's Vineyard anyway.
There are a couple of places on Martha's Vineyard where you can go horseback riding.
You can rent sailboats, take lessons, or charter a sailboat cruise on Nantucket. There are also a couple of seal cruise places on Nantucket.
When all else fails you can always join a clambake on the beach at night.
It's a really laid back place with nice people. The only place I've seen any problems is Hyannis. I came out of a movie theater there one night and several cars in the parking lot had their windows knocked out and things lying out in the seats had been stolen.
I go for the biking, beaches, seafood, antiques and crafts shopping, but especially the biking.