Working out a while at work

I have alot of free time, and sometimes I dont get my evening workout in. I do have time in the day where I can go for a good 15 minute walk, however this time of year is 100+ degrees outside. (excuses. excuses) Im looking for something I could do to keep active and not just play on the computer all day. Or do you know of any good sites that I could go to and get some ideas?