I can help if you are staying mid-lower Cape because I've lived here 50 years plus. Of course I know a bit about the upper Cape as well but not nearly as much and honestly, we avoid it whenever possible. The only exception to that rule would be a short visit to Woods Hole and all that it offers when going over to Marthas Vineyard.
The bike trail that starts in Dennis (Exit 9) is VERY popular and has some great little spots to stop and have a cold drink, ice cream, or even lunch. If you ride bikes at all, I'd take a couple of morning hours to do some of this
For beaches, the best beach in the Mid-Cape area is Mayflower. There are several very close to it that are nice as well. Be aware though that the parking lots fill often before 9:30 am

The bay beaches are best at low tide, very crowded at high tide (and I'd avoid that). When we can't make low tide at Mayflower we head to Nauset in Orleans. Great beach, great snack bar (be sure to try the onion rings), bathhouse, but can be very big waves...best for boogie boarding and fairly good swimmers! Teens love this beach. If you are staying further down Cape, Marconi and the surrounding beaches of the National Seashore are a must do!
Mid-Cape shopping/browing/lunch best bet is Chatham. The Squire has very good food and there's lots of little shops. Provincetown is a great day trip as someone already mentioned and the whale watches go out of there. We think whale watching is boring but if you've never done it, I would suggest trying it and keeping your fingers crossed to see some.
We have a drive-in theatre on the Wellfleet/Eastham line and if your kids have never been this can be fun for a cheaper night out. There's Red Barn Pizza just down the road that serves up great pizza/beer/wine.
Traffic can be a nightmare on the only highway that goes from the Sagamore Bridge to Provincetown. Try to avoid travel times, Friday night heading down, Sunday coming back....rainy days...etc. because it can be a real deal-breaker for fun.
A fishing charter out of Orleans or Provincetown can be fun...I'd do just a half- day or couple of hours or it can get boring for kids. You can rent jet skis out of Sesuit Harbor in Dennis and if you do, make sure to have a lobster roll at the Sesuit Cafe...a great outdoor seating BYOB seafood joint that us locals love!
If I can help with more specific information feel free to message me
