If you want a challenge...Baltimore. I've only done the half (well, the "baltimoron" - 5k and then the half on the same day!), but it's part of the greater Baltimore Running Festival (in all, they offer a full, half, 5k, the Baltimoron, and relays). Between the hills and the chewed up streets, it's a tough one, but the full course runs you through most of the city -- including the zoo, where they usually have some of the residents out for pictures. Prior to this year, you'd finish running through Camden Yards (home of the Orioles), but this year the finish was at the Inner Harbor. And the medals are pretty rad, too (the half medal opens to reveal the same image of Fort McHenry as the 5k). Come to Baltimore and get crabs! Have done some of the Rock 'n Roll events; last year I'd have totally recommended them, but since they sold the company a few months ago, everything appears to be rolling downhill a bit.