Some say "No Pain, No Gain" when talking about sports and muscle training. But suppose you just want to feel energetic and healthy.. pain should not be required. My idea is "train to maintain".
I can't go to the gym anymore and did not find any free weights to buy so I make do with large bottles of water (3 liters = 3 Kilo) which I use like kettlebells. They are used with a swinging motion to stimulate the upper body muscles and joints. I follow that with some pushups. And walk outside 30 minutes per day.