For anyone located in the Mid-Atlantic, Fairfax HS (VA) hosts an annual Division C invitational. This year was only its second year but teams from NC/MD/PA/DE (I think?) attended. It's a fairly laid back invite and the tests are good indicators of how much you've studied/need to study. In fact, the Dynamic Planet test was the second best DP test I've taken all year (Nats test was better but only because it had a greater emphasis on problem solving). I highly recommend it for anyone in the region!
Langley HS Science Olympiad '15