Coming into my third week of going for a short (really short, like 15 minutes) jog every morning immediately after getting out of bed, every day monday-friday. Awesome way to start the day for me, and while I've enjoyed running a lot in the past I've never been able to make myself run in the mornings, so bragabrag.

My recommendation has to be martial arts though, I'm a little guy with no aspirations of being able to beat up passers-by on the street, but just as a way of keeping fit and getting regular exercise, I found it super fun, interesting (there're different kinds of mental challenge there as well) and if you're lucky to get a good club then everyone pushes and supports each other which is awesome. I haven't done this for more than two years now but I spotted a local taekwando club near where I just move to so I'm going to try to get back into it.