I'm in Yarmouth, just west of Brewster and Harwich. You're right, riding here in high tourist season can be a challenge at times. I've found that the best riding days are also the best beach days. Many of the tourists are there, not so many on the roads. It's also good to take an early evening ride, when all those visitors are at dinner. Back roads that go through many towns, such as Setucket Road that starts in Yarmouth and ends at Rt. 6A in Brewster, are very nice and have little traffic. Tourists just don't know about them and stick to the main roads. Peace can be found, you just have to know where and when.
Actually, if you're here at Christmas time, the beach is a lovely place to be. Head down to West Dennis beach and just sit in the car and watch the waves. You won't be alone, but there won't be that many other people around. And there are plenty of other places like that, such as Bass River Beach or some of the northside beaches. Paines Creek Beach in Brewster is a nice brisk walk in the winter. Just you and the gulls. Winter here is an opportunity to see some beautiful things in a whole new way.