Trying to improve your brain is a bit backwards. You want to acquire and practice the most valuable skill you can. Doing brain training minigames is not the most valuable skill to learn.

It's probably one of the following:

public speaking/small talk (toastmasters)

connecting with your emotions (focusing by eugene gendlin)

a career or trade skill (trucking school for short turnaround, apprenticeship program in the trades)

typing (if you are below 50 wpm and write significant amouts, it's hours saved per day. If you write ludicrous amounts, then stenographic typing can help make the process flow better as it keeps up with your thoughts.)

communication (particularly nonviolent communication, there are youtube videos)

comfort zone expansion

I'm probably missing a lot of stuff that I've internalized to the point to where it's hard to imagine not having the skill. Even bringing up the last sentence reminded me of the skill of speaking standard or technical english.