The other option I live is bodyweight fitness. There's an app called Progressive Workouts that runs you through the Reddit /r/bodyweightfitness recommended routine and helps you build up your skills. Most exercises require no equipment, although a set of rings is used for a couple and workout bands help you build your pull-up practice.
There are also lots of prison workouts that require no equipment and can be done in a small space (no prison required!)
I'll try and block time in the morning, as it's easier for me to be consistent. If I'm stressed after a long day, will often do another quick workout to help reduce the adrenaline and cortiol built up by stress.
Standing knee kicks
Do these in variations and in sequences. Change it up a bit while also maintaining some level of structure to it.
Keep track of progress.
You can find videos of all lengths and it can be done almost anywhere.
With even very busy schedule you can always hop off bus or subway one stop earlier.
For when you're home, even if it's just for a few days, there are gyms that offer a sort of "stamp card" where you can use it for, say ten times. Those could be useful for when you travel often and with short notice. You could use it when you're home in-between trips. If you're at home for a longer period, you can get a gym card for a month and use it regularly enough that the cost is worth it and make sure to cancel the payments before whatever date the next payment will be charged.
Doesn't matter what it is – you can run just about anywhere and a bit of running gear takes up almost no space. If the weather is too frightful for running, you can do the 7 minute routine in any hotel room.