I agree with a lot of the posts here as far as wrestling goes. Probably one of the best "bases" you can get before moving in to other martial arts. One of my biggest eye openers after about 4 years of Tae Kwon Do where I'd earned a black belt in my teens, was going to a to a BJJ school where most of the other white belts were just guys who'd done some HS wrestling. I didn't stand a change against those guys at all. They had way better footwork, center of gravity, overall balance, etc and usually excelled quicker.