Due to being in the locality, I've visited New Hampshire Motor Speedway the most back in the early days of it being the Nextel Cup, and I also have some great memories of the smaller Seekonk Speedway, where my mother used to be on a race team.
It wasn't during a race proper, but when visiting Texas, I just had to go out of my way to check out Texas Motor Speedway, as well. (Gotta say, the people at the gift shop there are pretty cool.) I also have some memorabilia from seeing a Winston Cup race at Martinsville Speedway, but sadly, I was too young to actually remember the trip.
In hindsight, I'm very much seeing my NASCAR and local stock car bias show through.