Do a few extra squats every time you get up from a bed or chair

Do some lunges during a TV ad break

Chuck in a few half squats while you're waiting for your kettle to boil

Do a couple of elevated push-ups (where you support your body with your hands on a chair or a bench while doing the push-up) before tucking into lunch

Sneak in a couple of calf raises while you're brushing your teeth.