It depends on what your parents' interests are I suppose but I honestly can't come up with any reason to vacation on the Cape specifically, in winter. There is nothing in any of the towns on the Cape that could not be experienced in any other small town not far from the ocean.
I grew up in Falmouth and am there every week year round. I can assure you that it is just another small town. This answer pertains to Falmouth but much of it will be true for any town on the Cape.
I do persue some of my own interests there but if I had grown up in a different small town I could probably be doing so there too. Wandering in the woods and geeking around with my field guide and binoculars, birding mostly.
If your parents enjoy that type of activity they could have a blast.
Now if you plan a family trip in April there might be a bit more to do. Still, you'll need to post more questions concerning the activities the family might enjoy as a group and as individuals. In April most water/beach activities will be out except for beach walking but you might be able to charter a boat to take you out fishing (it'll be cold on the water). Golf courses will be open. You could go horseback riding. If you don't mind coughing up the money for a family membership, and if you own guns, you could skeet shoot at the Rod and Gun Club.
Most of the tourist-oriented gift shops will be closed. All the stores on Main St will be those that people shop in all year like the bookstore and the hardware store. A bit boring I would think. You can look at hedge trimmers and cans of linseed oil anywhere.
There are a few good to very good places to eat if you enjoy eating out (well, you'll have to) in town.
Up near the Villiage Green there are several old buildings, homes once, maintained by the Historical Society. You can tour some of them. Interesting if you enjoy stuff from the Colonial period.
Woods Hole, a villiage of Falmouth, is home to the Woods Hole Oceanagraphic Institute, one of the world's major marine research centers. I believe they have some tours of the facilities.
Woods Hole Harbor is picturesque. Fishing boats etc.
I cannot tell you how much it would cost to stay in Falmouth or elsewhere on the Cape because since my mother still lives there I have never had reason to rent anything.
You might be able to get an off-season deal on a cottage. One of the big real estate agencies is Harriet Dugan. If hers can't find you a place, noone's will. Google it.
I wish you would post more questions about this. I would like to help but I feel I am being depressing instead.