I try to always do some kind of continuing education, some career related, some hobby related.

The quarter-life crisis hit me pretty hard. I was in a similar situation, working too many hours, nothing much to do outside of work. I wound up enrolling in a grad program designed for working professionals. It forced me to cut back on some of the unpaid hours I was doing at work (are you salaried?), introduced me to a lot of new friends, and opened some doors professionally that might have otherwise been closed.

I like all these other suggestions above, too. If there is something you have always wanted to do or wanted, but it seems out of reach, start reaching for it now.