Dennis native myself, though I rarely return to the Cape.
As Steve said above, the Cape Cod Natural History Museum has a plank walk through the marshes to the bay, free to the public. Kids would like the museum but adults could cover it in an hour.
Not sure if it still exists, but I think it was called the New England Fire and History Museum, also on the north side of 6A (aka westbound lane of 6A), used to be awesome. Tons of visuals for people of all ages, old fire apparatus, old vehicles, tools, and more. I do not recall seeing it the last time I was out there however. There was a lower gravel parking lot, and you had to sort of walk uphill to enter the place.
Orleans Army-Navy, up 6A as well, on the north/eastbound side of the road, just past an Amoco station IIRC. Not sure if they still exist. An eccentric dude my father knew owned it. Used to sell beaten-to-hell Mausers, minus bolts, as kids' toys for $20 or so....wish I understood the potential project in those back in the day! You need to go to the second floor to find the good stuff, the first floor is/was all imported copies of BDUs, silly tshirts, etc.
If you want the full tourist experience, The Lobster Claw restaurant, on 6A (north/westbound side) in Orleans is decent food and service. Far from fine dining, but decent. Typical touristy-looking place with an attached fish market (that I have to admit used to be damned good).
Bird Watchers' General Store, same stretch of 6A in Orleans, was always good for a walk through once a year or so. Tons of odd gifts, something for everyone, not just a place for bird fanatics. The owner's sense of humor plays well into much of the store.
I hate to preface everything by saying "if it still exists", but that's the case! Cinnamon Rainbows, if you like older-school surf and skate culture, used to have a mix of new and used and NOS items from that culture, plenty of which was/is real vintage stuff, on the Orleans/Eastham town line.
If it's not closed for the season, Cobies, on 6A by Nickerson State Park, has pretty good greasy fried food and typically is staffed by lots of scantily-clad female eye candy. Nothing but the fried seafood is good however, they can't even do French fries properly. But the bubbly college girls make up for it :)