Video Games.
I'm good at some of them, bad at most others, and I sure waste a lot of time playing them.

Here are some non-steam games that I've thoroughly enjoyed. Playtime is very roughly estimated.

1. osu! [osu.ppy.sh], 1000+ hours
2. GTA 3, VC, SA, LCS; all to 100% completion, 400 hours
3. All PS2 Tony Hawk games, 250 hours
4. Final Fantasy VII, 210 hours
6. Doom 1 & 2, Quake, 200 hours
5. Mario Kart Double Dash, 150 hours