#1: Calisthenics is your friend
Pushups, handstands, pistol squats, wall sits and tricep dips are very doable with no equipment if you get creative about how you perform them. You can easily look online to find different articles on how to do each of these exercises.
#2: Carry some resistance bands
Resistance bands are an awesome way to create tension within the muscle to keep it stimulated for muscle maintenance. They are incredibly cheap and space efficient. When you decide to buy some, a simple online search will show you multiple ways on how to use them.
Some form of cardio is very easy to do in an unknown place while traveling. Whether that is renting a bike to explore a city, kayaking across a lake or strapping on some trainers to your feet and hitting the road for a run.
#4: Find a gym if you can
The prior options on the list are useful but are secondary if there is an affordable gym near the area in which you are traveling. Always scout to see if there are gyms in the area, if not you should have those trusty resistance bands you packed earlier!