We live in MA and go that way every year for at least a long weekend so I have lots of them:
Depending upon budget one suggestion is heading up to Chatham and doing a seal tour - expensive but supposedly really worth it - last year there were lots of seals and lots of sharks.
One of the things we've done a couple of times is taking a tour of Cape Cod Bay on a schooner boat - the Bay Lady II is the one we've been on. You can do it in the morning, the afternoon or a sunset cruise (the sunset cruise is BYOW or BYOB) and fairly affordable. My whole family went last year for less than $100. I have photos I can share if you want to see them.
If you are not afraid of heights then climb the pilgrim Monument. There's also a museum of Provincetown history there.
Just go shopping - there are lots and lots of stores. One suggestion - if you have kids of any age with you AVOID Purple Haze. In addition to various other items it's primarily a sex toy/drug shop. There will be all kinds of paraphenalia you don't want to have to explain to a child. ETA - I was cringing last year watching a woman with a 4 yo come running out of there. The store has this neat really colorful exterior which appeals to kids.
Dinner or Lunch at the Mayflower. Really good homestyle food. My husband loves their burgers and clam chowder. We have to go there everytime we go to the cape on vacation whether camping for a weekend or in a cottage for a week.
You could do a Dunes Tour - again this one is pricey.
Get a copy of the National Seashore guide from the Provincelands Welcome Center or the one in Eastham. There will be tons of activities listed there. Some of them are free or really cheap. One of our favorites is to do the Nauset Beach Fire (in Eastham at Nauset Beach) hosted by the National Park Rangers. Another favorite is to see the Coast Guard Rescue Demonstration.
Not so much a must do but you could also go on a tour of the now defunct Air Force Base in Truro (Next town over)- again run by the Park Rangers. There are some amazing views of the coast from here.
Also, in Truro you can go for a tour of a lighthouse and if you have good lungs/heart/legs you can climb to the top.
ETA - you could also hit a drive-in move in Wellfleet or the flea market at the drive-in on Sat or Sunday morning. There's also lots of places along Rte 6 to go mini-golfing, get ice-cream, etc.