Make little changes in your daily routines, so that your brain with have to think a little more about what you´re actually doing, instead of just running in default mode. Small things like taking a different route while commuting can prove very stimulating.
Reach back into your memories. By looking at old photographs or other media you might have, let your brain dig into its memories and take you back in time. This is a great mental exercise. If you want to increase the performance of your brain then you can browse https://www.thebusinessbrain.co.uk/.
Enjoy good music and art, and if possible learn to do it yourself. Playing an instrument, for example, is a tremendously powerful workout for your brain, because music is a global process that implicates all parts of the brain in a holistic way.
Learn new words and even new languages. You don´t need to be proficient, but introducing your mind to a few new words a day will expand your mental horizons and keep your neurons working on something new all the time.
Close your eyes and visualize yourself in some other place. Visualization is an intense mental ability that can be developed greatly by frequent practice, and will enhance your abstract thinking process significantly.