Running. 45 minutes, 3 times a week.
I'm a big guy, but it only takes a few weeks of jogging before you can run 4 miles without stopping. I used to lift weights but i found that hard to keep focused on and be dedicated to. Plus the fitness benefits aren't really there.
Jogging outdoors is much more fun than a gym. You have to deal with hills, crossing the street, pedestrians (Neds in particular)... and ofcourse the weather, but that just adds to the benefits. With a running jacket on you can still be wearing shorts in the rain and snow. Its more of mix than just a running machine.
Jogging is pretty easy to get started, all you really need is an old t shirt, pair of shorts, and a reasonable pair of trainers (which you may need to risk 10 minutes in JJB sports for). A digital stopwatch is handy, and an MP3 player provides the musical motivation.