Sitting at a desk (or just sitting in general) for long periods is not conducive to a healthy lifestyle, but for many office workers, there isn't another option. Working at a standing desk can help alleviate some of the back, hip, and neck issues that can come up because of sitting (I have found standing while working to be effective for me over the last year), but essentially, other than your hands or arms, your body is still predominantly motionless.
One option could be using a pedal unit that fits under your desk, allowing you to pedal at varying levels of resistance while seated, which can help mitigate some of the effects of sitting, while also helping you burn some calories and feel a little better about that third or fourth donut from the breakroom. But if you really want to get the most out of pedaling at your desk, then it makes sense to convert that mechanical motion into usable energy.
Whether you want to keep your legs moving to offset some of the effects of being sedentary, or you want to be able to burn some extra calories while at work, or you just want to be able to charge your gadgets without having to plug into the fossil-fueled grid, this compact and portable pedal-powered generator from the folks behind the hand-cranked Pocket Socket generator might be worth looking into.
The Power Box, from K-TOR, allows users to plug in any mobile device with its own charger and to top off its battery by pedaling, and while it's best suited to smaller devices (phones, MP3 players, tablets, etc.), larger electronics (such as laptops) can also be powered with this generator.
"The Power Box can produce up to 20 watts. It can directly charge a full range of portable devices. Because it is so powerful, it can charge more than one device at a time — like a smart phone and a tablet. Since it can be cranked for long periods of time comfortably, it can be used for higher power applications that it cannot charge directly. Simply charge an external battery and run your appliances, either directly from the external battery or through an inverter."
The Power Box folds down to just 12" x 5.5" x 3.5", and weighs about 4 1/2 pounds, so it's small and light enough to be carried to and from work, or moved to different desk, and could also make a great addition to an emergency preparedness kit or for powering devices on an offgrid adventure.