1. Eat reasonably and keep your body fat down. Diabetes and prediabetes are big risk factors for cognitive impairment throughout life.
2. Maintain at least moderate aerobic fitness. Strength training won't hurt, and may also be beneficial.
3. Don't neglect balance, speed, and flexibility training as part of your physical exercise, because these abilities will help you guard against falls and other injuries that are actually very big considerations as people get older.
4. Keep your mind active. Keeping your mental muscles exercised seems to be far more than just an expression. But you have to be engaged. Do things you actually like rather than halfheartedly doing things you don't care for at all. So you don't have to do crossword puzzles or computer brain games if you don't want to. But do something.