I stayed at Nickerson last year, and you need a week to properly explore it..... Hell I still feel like I missed things. P-town is neat and we spent the whole day there and did a whale watching trip that my 4yr old really enjoyed. the National Seashore is worth a stop, Martha's Vineyard was a whole day and completely worth it, and while not on the Cape, Plymouth is right down the road and good for another whole day. Be prepared to see a lot less than you think you will accomplish because traffic is a nightmare everywhere. There is literally one road in, and one road out of the Cape and they are constantly packed.
I also forgot.... Good luck parking a C class in Ptown. It was hard enough finding room for my F250. You have no idea how many times I heard "I don't have any spots big enough for that" from the people that run the pay lots.... This happened to me all over the Cape. You may want to consider renting a car if you don't have a toad.