I've always been a fan of creating content for video games. I can't say I'm that good at it but it is fun. If a game came with a map maker or scenario editor you could be sure that I was going to play around with it. These days it's getting even better since many of the source engines are available for free! That's right...you can mess around with the actual tools used to make the game. It's neat.
My biggest accomplishment to date was a fully functional map for Team Fortress 2. I loved the cartoon look to the game and it made making the map easier since you could get away with less detail. It was a bit ambitious and didn't work all that well but I got some positive feedback from some players. Apparently trying to render an ICBM in the middle of the map was a bad idea. Who knew?
I downloaded Unreal Engine 4 recently which is amazing and free. You can do a new game from scratch or load it to do just Unreal Tournament modding. I also found out that CS:GO still uses the old Hammer Editor for map making. This was a bit surprising and fun to load up again. I couldn't remember how to do much though which was disappointing.
For an easier project I found there was a toolkit for Civ V which was easier to navigate. I made some silly maps and had some fun. There's a huge range of tools available and I sort of want to try them all. I should have been a game developer.
Anyway. If you've never looked into it, look and see if your favorite game has an SDK or a editor. You can extend your enjoyment of any game by learning how to make custom content. Plus with the Steam Workshop and other distribution systems it's easier than ever to get your creations to the masses. If you're good you can even make a little money. Not bad.