Exercise. It does wonders for your energy levels and your mental well-being. Plus it will make you feel you having done 'something useful' by the end of the day. The hardest part is getting out and starting - once you are busy, it is fun.

Otherwise, reading. If you have enough energy, read literary classics or non-fiction, the type that challenges you and/or teaches you new things. If you don't have enough energy, read something lighter - I am reading Alice's Adventures in Wonderland currently, for example, which is fun and relaxing. Finishing a book is a nice feeling, plus the mental focus on the story and on yourself can give rise to some nice insights on yourself, people, and the world.