Non-Disney Family Trip - Suggestions needed!

My dad's big 7-0 is next year, and as a surprise we want to all go on a family vacation together. It would be the first time really we all have gone somewhere together as my brother was out of the country for a while with his girls. It's also my sister's 40th (they were born exactly 40 years apart) so it's a big deal for the family.
The actual birthday is in March, but i spoke with my SIL and she doesn't really want to take the girls out of school so we're looking at a summer trip. I've been trying to come up with some non-Disney alternatives to suggest. Now, I'd love to go to Disney with them all but money is kind of an issue, and honestly I think my Dad would rather get a house somewhere that we can cook big family dinners and play by the pool all day with his GD's. Also, I don't think my bother and sister really want to take the girls on a plane again any time soon - they just moved back from overseas, and a 14 hour flight with 2 kids and a toddler was more than enough for a few years!!

Here's the cast of characters:
Dad/Mom - 70/67
Bro/SIL - 40's
Sis - 40th
Me/DH - early 30's
Nieces - 10/9/4

Any suggestions of somewhere on the East Coast, in easy driving distance from NYC that would have room for all of us (at least 4 bedrooms)? We were thinking of maybe down to Williamsburg VA for the 4th of July (I'm guessing it would be crowded, but he'd love all the celebrations). Or maybe somewhere in the non-Snooki areas of the Jersey shore...

I'd love to find someplace that was good for the nieces to go and play and have fun, but still have some places for me and my sister to go out and have some adult times - it is her BD trip too. I'm not really a beach person, so I'd like the idea of having something else to do besides that...

I know it's more than a year away, but we're trying to plan and budget now, especially if we have to book early to make sure we get what we want.