New England Summer Vacation Help!?

Does anyone know of any nice places in the Cape Cod, Nantucket, Martha's Vineyard areas that are cheap but nice for a young couple looking to vacation in any of those places? My boyfriend and I are 23 and 21 and we wanna stay somewhere on the beach in any of those areas but for no more than $115 a night. We're planning on staying for 3 nights (Thurs-Sun) in July 2009. Also, we must have a private bathroom, no shared bathroom please.