Like kidzer, Bob Smith and JamesF1, I used to either walk everywhere, or be heavily involves in some sort of sport during until about the age of 19. Since starting to drive, I've noticed a marked decrease in my general fitness.
So, as part of my New Years resolutions I started a fitnes routine; primarily using my own body weight. Daily I try and do;
5 minutes of "warm up"
5 minutes of hindu squats
5 minutes of hindu push ups
5 minutes of bridging
As long as I can now manage doing regular push ups
Warm down
I'm specifically not trying to loose weight, but to increase my upper body strength and general fitness. I will also try to use 2 small-ish barbells at least once a week.
Unfortunately after all of this for about a week or so, I'm now being accused of having a flat arse. I have found that my upper body strength has significantly increased, and from what I'm assuming is the briding, less of an achy back after being at the computer all day.