Actually, physical activity is a big boost. Increases blood flow to the brain and muscles, and relieves tension and stress. I find that my productivity increases with less effort when I'm more active - and even when I'm not, I take the dog for a couple of brisk walks in the park not far from my place.... amazing how much breathing fresh air can do...
The other thing that I find increases productivity: inertia. Keep yourself busy. If I stop, and take a day off, lazying around etc, it's really hard to get up to speed again. In other words it's easier to stay productive than to start being productive. It doesn't have to be one dimensional - there are lots of things to keep you busy: household repairs & chores, errands, computer stuff, keeping up with family/friends (yes, it's a tough job! ) as well as professional/academic activities.