At home I enjoy spending a few hours each week lifting weights. Long-term travelers do not have access to this convenience. Instead create a routine with what you have.

Plan on using your main backpack as the weight (resistance bands work too). You can wear your backpack for pushups, squats, etc. Or you can grip the top handle like a dumbbell for curls and shoulder raises. When you need to change the weight of your backpack, just increase or decrease the stuff inside. For added weight, fill your bag with 2 litre water bottles.