Movement promotes cognitive performance. Either take breaks and exercise, as you already do, or set up your reading in a way that allows you to move while you read:

 - take your book or paper in your hand and walk around in your room or garden while you read, or take it to the park; here is a how-to, if you need one: http://www.wikihow.com/Read-While-Walking, and you can find a lot of essays on Reading while Walking through Google
 - some books are available as audiobooks and easy to walk or exercise to
 - buy or build a standing desk and stand while you read or type; even though you don't move around much, your posture is more healthy and you need more muscle to stand than slouch, so you are in fact active in a less obvious way
 - sit on a big exercise ball; similar to standing, this induces you to move around a bit more and use more of your musculature; but keep your regular chair and move back to once you get tired, because slouching on an exercise ball is even worse for your back than on a chair with a supporting back
 - knead something with your hands; there are special balls and other objects for this, like therapy putty, hand exercise kits etc.
 - chew gum; see http://cogsci.stackexchange.com/questions/4299/does-chewing-gum-mentally-help-basketball-players-make-foul-shots