Calisthenics are bodyweight exercises. Most people mistakenly believe these are all about cardio and high-reps, but they don't have to be.
There's a great book (well, series) called Convict Conditioning. The author lays out a set of six basic bodyweight exercises that target the major muscle groups:
- Handstand Pushups
- Leg Lifts
These six exercises are easy to perform quickly at the start of the day. I recommend a regimen that incorporates all six exercises - one set each, to failure. Even if you can't get to the gym, these exercises are easy enough to do pretty much anywhere (you might need a bar for the pullups...).
May the Grace of the Lord Jesus Christ be with all God's people. Amen.