Originally Posted by bystandering
You won't lose much in 3 weeks. The small amount you will lose will easily come back to you in a week or two.
Regardless, you can attempt to maintain your muscle mass with some bodyweight exercises.
Pull-up - One of the best if not the best back exercises. Add weight by tying something to yourself or grasping something with your legs.
Push-up - Widen your grip to increase difficulty and target your chest more
Bench dip - Targets your triceps
One-legged squat - Do not underestimate this. Do a few right now, lightly touching something if you need stability. Use a full range of motion. Your legs will feel it if you've never done this before.
Single-leg deadlifts - As describe above. Good for lower back.
Crunches - Great for strengthening your core
If it gets too easy, increase the length of the contractions or negatives, or play with the speed and reps.
I don't think you'd need anything else. Get back to the gym though. Your own bodyweight can only give you so much resistance.
To tag on to Bystander, there are literally dozens of types of push-ups you can do (incline, decline, one-arm, etc) not to isometric training (muscle resistance against itself at a slow, excruciating pace). Things like squats against the wall --go from standing to a seated position, hold for 30 secs, then slowly raise up. This will really burn.
Calf raises work without weights also. All are
I might suggest bringing a weighted jump rope also. Increased cardio and strength. One of the best all-body exercises you can do. It'll keep you toned while you travel.