If you are over 50 start slow and if nothing else just walk
Do yoga or Tai Chi to gradually build flexibility and boost muscle tone
If you have back problems do Pilates exercises to rebuild core strength especially for the abdomen. I know cat & dinghy sailors who gave the sport away because trapezing was agony. I last about 10 minutes but am working on it.

Hide the TV remote or put it where you have to stand up to get it.

If you are an office worker get up and walk around and stretch every 20 minutes.