this is the easiest of all exercises.
You can do it anywhere, inside, outside, alone, with a friend. Go for a long walk or just around the block. (and I don't mean the toy block on the floor!)
If you are out of shape it is a great way to get started exercising. Go to the corner and back, when you are used to that go half way around the block and back, when you are used to that go further. Before you know it you are walking all over the place and actually enjoying it!
There are some great stationery walking tapes out on the market if you want to stay home because maybe the baby is sleeping.
The most important thing is that you do it!