Off the Cape
There is so much to do on the Cape, you really don't need to leave, but if you're restless or the weather isn't great, there are incredible places you can visit within 90 minutes.
Newport
Providence
Boston
If you are local, there is no need to drive up to visit Boston, but if you're visiting Cape Cod as part of a trip to New England, Boston is an easy day trip as long as you time the traffic. You can take advantage of everything the city has to offer and even catch a Redsox game and get home before midnight. If you don't want to drive there are fairly regular buses from Hyannis to South Station.
Newport, RI is also only 90 minutes away and you can easily fill up the day with shopping, dining and visiting the various attractions. The gilded age mansions from the Marble House to the Breakers are truly unique as is strolling on the cliff walk or driving Ocean Drive.
Providence is a great option if you want to get off the beach and out of the sun, but don't want to deal with Boston traffic and parking. If you have kids who like zoos, the Roger Williams Park zoo is terrific. It is a compact city zoo, but really well done and our kids loved it. Check the schedule for "Waterfire" pictured above. On a few nights in the summer they light fire pits and have live music throughout downtown Providence. Finally, there is a vibrant food scene in Providence that is a little less chaotic and expensive than Boston and NYC.