Reading is the best simple advice.

Reading will keep the mind agile because it

• builds empathy, you will be less likely to disregard other ways of doing things
• gives perspective
• is an active way to learn which makes learning other things feel less arduous. In other words you are giving yourself one less reason to become jaded

The reasons go on and can be explained better. I recommend finding out about it and making reading a habit because the benefits, while not immediately obvious, are incredible and everyone should do it.