Update: Sold out.
Your hands do a lot for you — why not buy them their own home gym? This set of exercise putty from CanDo includes six 2 oz. tubs of goo (in fun primary colors, no less), and unlike an elliptical or a set of dumbbells, it only costs $12. Playing around with the putty is basically a form of resistance training for your fingers. And not only will it strengthen your muscles and improve your fine motor skills, but it also can act as a big gooey stress ball.