This is the worst weekend for that sort of thing. Everyone is on vacation. Think at this point if you wish to book a vacation weekend on the Cape, it should probably have been booked in early spring. One of my techs from NJ tells me that the entire Cape May area has been booked solid since March, because people who used to go overseas for their vacations have decided to stick around and do cheap stuff.
Seriously, I commute to Cambridge, taking the MassPike, which is normally a 60-90 minute drive in rush hour. This past couple of weeks, the "last family vacation before school starts" weeks, it has been 35-45 minutes instead because everyone's on vacation.
Have you thought about going to the North Shore area? They've got beaches too. Some cute little towns with clam shacks and lobstah rolls and all that. That's where DH and I usually go for beach days, there are some nice bed and breakfasts although you might be outta luck for pools.