Hi there,
There are lots of things you can do that don't take up much time.
Im a personal trainer and give lots of my clients quick home workouts to do while their baby is asleep or otherwise occupied and you have a little bit of time to yourself but can't leave the house.
Here is a workout to get you going and give you an idea of the type of thing to do...
5 walkouts (touch your toes, walk your hands out until youre in a plank shape, then back up again)
10 squats
10 alternating leg lunges
10 crunches (feet on the floor, head in hands, look up and tuck your chin into your chest, hold for 3 seconds and then release slowly)
10 legs up crunches (exactly the same process as the previous exercise but keep the legs lifted off the floor this time)
20 V Sit twists (sitting up, lean back with heels lightly on the floor and slowly twist placing both hands on the floor either side of you)
Try to do this 3-5 times all the way through and you should feel like your stomach (and the rest of your body) has worked hard!
Try to do this as often as you can throughout the week and you should start to notice a difference. Don't rush the stomach exercises, the slower the better!!
Good Luck,
Charlie Launder
http://www.charlielaunder.com