One of the easiest and most effective exercises is running because it gets your heart rate up and burns calories.
A good way to run is on a treadmill. You'll avoid car fumes and be able to run, regardless of the weather. You can also get accurate readings of your running progress.
The only problem is most people find treadmills boring. But they don't have to be.
How to make the treadmill fun
Get a friend on board
If you both belong to a gym, run next to each other. You can motivate each other and provide distraction so the time goes faster. Running with friends works better on a treadmill as you can both run at your own pace, which you may not be able to do if you ran outside.
Try interval training
More evidence is coming through that this is a better idea for calorie-burning than straight running. Basically you alternate between hard and easy runs. Try a 5 minute warm up (walk), followed by 4-5 sets of 4 minute runs split into 2 minutes of hard running and 2 minutes of easy running. Finish with a 5 minute cool-down
Don't do it all at once
After you've been on the treadmill for say, 10 minutes, hop off and lift some weights. Get back on for another 5 or 10 minutes, then get off again and do a few sets of crunches. See the pattern forming? Mix up the rest of your workout with lunges, dips, side bends, leg press, etc.
The beauty of the treadmill is that you don't have to be alert to what's around you - like cars. You can just run mindlessly. Try taking yourself inward, switch off the brain and concentrate on your breathing. Or plug into your iPod and drift away. You may find you run further without even knowing it.
Swap between machines
If your gym has an elliptical trainer, bike or rowing machine, swap between them. Start off with 10 minutes on the treadmill to get warmed up and then start doing 10 minutes sets on each piece of equipment, moving back and forth.