Various activities/games with words, numbers, visual patterns etc. Will it boost your brain power? I doubt it. But those are things that you can use to keep your mind active, have fun and learn/develop your brain at the same them. Don’t expect extraordinary result, but this won’t hurt of course and can particularly be helpful for children. It’s like all those brain training apps that we see, except on paper, it’s better than nothing, just don’t hope for miracle here.