babymoon outside of Boston?

T
My hubby and I will be attending a wedding in Boston in August and since I'll be almost 6 months, we want to make a babymoon out of it. I've looked into Nantucket and Martha's Vineyard, but holy hell those places are expensive. I've also looked at Provincetown which is beautiful and more affordable, but I've been there and would like to go somewhere I haven't been yet.  Anyone been to a beach town that's romantic up in the cape cod area for $300 or less per night? And it'll be high-season prices since it's August. Would be great if it's somewhere with a spa nearby so this mama can get a massage. Thanks!!