Doing simple stretches before your flight is probably best but honestly, do whatever! Do jumping Jack's in the middle of the airport, don't care what others think of you! Aslong as you do what makes you feel happy! If you don't feel comfortable doing it inront of people, try and find a quite place where there aren't many people
Go into a bathroom stall for some quick yoga stretches. Or use my phone and power walk up and down and get my steps up. If your mobility is low, you can Google yoga for low mobility
When I am traveling I focus on the EASIEST exercises possible: the ones that require zero excuses. I will jog through and airport (pretending to be one of those people late for a flight), I will explore a new place by going for a walk instead of just driving, or I will find space to do a quick HIIT workout. Always pack workout clothes. Always plan 30mins of movement into your day.