You'll never go wrong with pushups.
I recommend you integrate BW exercises using a superset format that you blitz through multiple rounds, trying to complete as many rounds as you can within a fixed time period, for example 10, 15, 20, 25, or 30 minutes depending on how much you you have available and your current fitness level.
Let me suggest the following simple circuit. Try 10-15 minutes in the morning and 10-15 minutes in the evening, 5 days a week.
10 pushups, 10 BW squats and 10 situps = 1 round. Complete as many rounds as you can within your designated workout period, the more the better.
If these rep numbers do not match your current fitness, modify them.
For more ideas, hang around the
Online Journal and see what others are doing.
Keep us posted how you are doing. Have a great day!