Rock crawling. As mentioned by Esheep, it adds up quick. $800 for two seats and saftey belts. $3000 rear axle. Not to mention that we carry spare parts that most people have to go to a mechanic for. A major repair for most is usually occomplished with a rock as a work bench, a pair of vice grips and hammer. I went out the day after thanksgiving and locked up a front brake caliper. I pulled it off the rotor, zip tied it up to the frame with a piece of steel in it to keep the piston from popping out, then continued on. About 2 hrs later, I blew up both front axles. I pulled the drive shaft and followed the rest of the group to watch from there on out.
I guess it can be considered separate but related, but I'd call the building and repair of jeeps as a hobbie also. Welders, tubing benders, 80 gallon 2 stage air compressor, air tools. That list goes on farther than the list of parts on my jeep.