Try as many sets as you can accomplish in the time you have (so do an AMRAP--as many reps as possible--in 20 minutes, if that is the amount of time you have available).
1.) Abdominal holds: Pick yourself up from the table and hold your legs straight out for as long as you can (shoot for 10-15 seconds).
2.) Up-down planks: Hold plank position and drop each arm to your elbows and get back up, one arm at a time. Do this as fast as possible within your time goal (try 1 minute).
3.) Upright rows: Using your small duffel bag of heavy items (as explained in an earlier post), get into a good squat position, as pictured, and keep your back straight. Lift the bag straight back from starting position, only engaging your arms and upper back. (Do these for 30 seconds to a minute).