Me - I would do pushups and chair dips every night(or morning) - Just use your body weight.
Start with something comfortable - Say 2 sets of 20 pushups tonight and then every night add 1 more - every night - you are not building muscle size - just stamina.
Chair dips - 3 chairs - 2 that are about shoulder width apart + a little and the 3rd that you rest your heals on. Again so something comfortable maybe 2 sets of 20-25 and then add 1 to each set each night.
I am sure some people have better ideas, but what I like about these are you can do them anywhere