I got into HEMA (Historical European Martial Arts) fencing last year through a club in my little town. Olympic/sport fencing is fun, but imagine (safely) swinging a 4lbs. steel longsword with two hands at your opponent instead. It's a ton of fun, a great workout (I burn ~1500 calories per class), and competitive so it keeps my interest.

Then there's the whole nerd layer of reading all the original sources from the 15th century, attempting to retain the historical character of the techniques while engaging in real combat, etc. It's both intellectually and physically stimulating.