There're a lot of fun things that can positively to your mental and physical health. If I had extra time, I would try and learn a few trades like blacksmithing and wielding. There're a few languages I'd like to learn. There're some musical instruments that I'd like to learn as well. We were designed to work and play outside, not sit at a desk for hours on end. People that do stuff regularly outside generally are healthier from the activity, vitamin D production, and breathing some fresh air.
Start a side business that doesn't directly compete with your employer if your downtime isn't still on their time. If you do go this route, structure your company in a way that removes you from any liability; you may need to find a good lawyer for that since there're other good lawyers that are adept at legally seizing the company owner's assets that aren't related to the company. A former colleague of mine started a site that he sold fantasy and reproduction props from. He didn't maintain any inventory and just dropped shipped from suppliers. He'd advertise his side on a couple of enthusiast forums that he engaged in and also did SEO work on his site to get at the top of Google results. He said he spent one hour a day working on fulfilling orders with the suppliers, answering e-mail, SEO, and engaging the community. He ended up with about $2k/mo in profit.
If you're not interested or can't do that, try finding a fun video game. I suggest the X series from Egosoft. Those games are a blackhole of time. The latest one is X3: Albion Prelude.Each new release has gotten better and you really don't need to play the previous games in order to play the current one, however the back story from the previous games is interesting and explains WTF. You can still read the plot on Wikipedia IIRC. Another option is to play some MMO. Grind away and sell virtual wares for real currency.