You mention doing situps here, another exercise that is really good for any type of cleansing is intestinal exercises. They get everything in the intestinal tract moving and you can do them anytime. I usually do them in the morning or on the way into work if I am running late.
You do them by breathing out and pulling in your belly button and imagine it touching your spine. Then breathe in deeply, extending your abdomen at the same time.
It helps you to focus if you put your hands over your tummy with your thumbs and forefingers touching in the shape of a diamond. It is actually the point two inches below your belly button you want to focus on.
If you do these first thing in the morning you will find it helps to encourage morning time bowel movement as well as starting your day with increased vitality.