I've never figured why so much emphasis is placed on upper body 'vanity' excercises like sit-ups and push-ups for losing weight. Your legs have the biggest muscles in your body and hence are the biggest source of burning calories. So, if your goal is simply to lose some extra weight and you have your diet figured out, then excercise that involves your legs will produce the most results under the shortest time.
Get a road bike, and go out and pedal every other day. Ride hard enough to keep your pulse elevated and breathing strong but steady. It's a good way to get fresh air, meet chicks, and it won't detroy your knees and hips like running will eventually do.
Take the remainder of your time and do some lower back / ab excercises and some range of motion work on your shoulders with a pair of cheap dumbells. You'll feel great in no time.
I'm also opposed to monitoring every once of body mass you burn off with a scale. If you are excercising right, and eating right, there's no need for the 'biggest loser' approach because you'lll lose weight. Let your body and motivation decide how much weight to lose - not an appliance form Walmart.