I don't want anything complicated. There are already enough challenges and constant responsibilities in life. I want something that's like chewing gum for the brain, just a little more useful.

Sounds like you want everything at once. The truth is, you're already surrounded by all kinds of activities, you just have to find them. And nobody but you knows what you'll actually enjoy the most.

Brain teasers, simple games, reading books, puzzles... There's honestly so much you can do. Speaking of puzzles, that's my favorite. I've loved putting together jigsaw puzzles since I was a kid, and now I mostly do them online at https://magicpuzzles.net/, where it's free and the pictures are top quality. Puzzles are like chewing gum for my brain, but without doing any harm to me or my mind.