Combat Sports.
I got bored of the gym, so I decided to take BJJ. Grappling is really fucking hard, as in you have no clue what you're doing, and no idea how to stop someone from fucking you up on the ground. It's one of those sports that you can spend six months doing and barely get a feel of wtf is going on.
Two years later, I was somewhat capable, and got my blue belt. I then noticed that I was actually pretty good compared to the white belts. Things started to make sense, and while I got absolutely fucked up by everyone else, the positions made sense. I'm now a purple belt, and the other day I did an iminari roll and a rolling guillotine on a white belt during a spar, just because I could.
In the middle of this, I started doing MMA. Striking is also hard, especially when you mix with wrestling/grappling. I came in as the guy that was fucking useless with striking, but when we took 45 mins to do some grappling the coach was wondering why the new thirty-something idiot was tapping everyone. Eventually we found my level, and he gave me some solid pointers on how to work on my striking to bring it up to level with my grappling.
All in all, combat sports seem pretty scary, but getting into it is just a matter of turning up and giving it a try. You'll feel like a useless idiot for months, but before you know it people will be asking you wtf you just did to them...right after you had had the same conversation with the person that's better than you.