I'll add in my support of Rehoboth or Lewes, we go down there most every summer (quick MHT->PHI flight). I generally stay in Rehoboth, but for most of my beach time I go up to Henlopen in Lewes, as that is one of my favorite beaches (and if you want a break from the beach itself, there's some nice hiking trails and the old towers from WWII)

Rehoboth has some pretty decent restaurants, too.

When he lived in NoVa, my brother would often come out and join us, but he did have to take care to avoid peak traffic.