A general tip for honeymooners from Lonely Planet's Reid: "Try to avoid honeymooning in June or Summer. If that's when you get married, plan your honeymoon for the shoulder season of September, October, or November. Many, many destinations are cheaper. Even September works."
"Beach rentals in Martha's Vineyard can still be enjoyed, and prices drop by 50 percent after Labor Day. So I'll say September, Martha's Vineyard — get around on bike trails, dig for clams, open wide beaches you can still swim in, and cheaper prices after kids go back to school. It's a two-week window or so before it gets too cold."