An interest of mine has always been to combine workouts with technology, and this is one of the ideas I've had.
I've always felt that working out at a gym should be integrated with the world around us, and what better way to do that then connecting it to the power grid? This is the Power Dumbbell, simply a regular exercise dumbbell which is connect to a dynamo and a battery. This can then connect to your buildings power supply so that you can later use the electricity you've generated. Or why not connect it directly to your laptop as an incentive; if you want to sit still all day you've got to move your ass first! Of course the weight of the dumbbell is adjustable.
Some quick calculations showed that this device could roughly generate 5-10 watt in normal use (see below). It's not much, but it's a start. Here's a quick render I did to give you an idea of what it would look like:
E = m*g*h
m = 10 g (mass of the dumbbell)
g = 9,82
h = 0,65 m (the height the dumbbell is lifted)
E = 10 * 9,82 * 0,65 = 63,83 Joule
If the user does a normal 10 times 3 workout (does 10 lifts, rests, then 10 more etc.) in a 5 minute period that gives us:
t = 60 * 5 = 300 s (time)
E = 30 * 63,83 = 1915 J
P = E / t = 1915 / 300 = 6,38 Watt.
Of course this is just one hand, I normally work out the other hand as well in those 5 minutes, which would double the electricity generated.