Walk lots... 30 mins a day, at least 3 days a week, make it a brisk walk and you'll notice more of a difference. Download a free yoga guide from the internet/get a book from the library and start do some yoga maybe once or twice a week. Do situps when watching tv/listening to music. Walk during your lunchtime, even if it's just round your office. Maybe you could clean up an old bike or borrow one and use that. You could always try jogging/running, and skipping if you already have a rope. There is a lot more things that you could do, but I was trying to stick to the "free" part!