Run or walk the stairs for 20-30 minutes on your lunch break.
Take a stroll around the office once every 45 minutes. Pretend you need to talk to someone about something important (TPS report!) or refill your water bottle (only fill it halfway each time and force yourself to get up more). Be consistent and frequent!
Perform seated marches at your desk. Push your chair away from your desk (for leg clearance). While sitting upright, lift 1 leg at a time for 6-8 seconds (alternate). Repeat 10 times.
Find a place to stand and work on your project. Naturally, your heartrate will drop while sitting. Stand up and work on a project!
Dance with your boss. Check with your human resources department before doing so.