Running is an all time good one
There are tons of bodyweight workouts, but I never can seem to do them. Seems so boring.
Maybe instead of running, if the weather is crappy, just have the clothes to run in the rain and bad weather.
Or bring a jump rope?
Another alternative is to go to a gym. If there is a gym in the hotel that is really nice because you can just use their stuff.
Theoretically you can use the gym if you have a nationwide gym membership. Membership is fairly cheap as well.
But a gym is boring.
Another way is to like go to an activity gym. I guess like climbing or martial arts gyms would be a way. Any kind of activity that you do.
A third option is to bring like bands in your luggage. They take up very little space and very little weight and can provide a lot of resistance.
Another all time solution is just to not workout. On vacation you are typically swimming, biking, and walking all over the place. Count that as workout.
I guess you could bring like a weight, but that seems like too much. Traveling by plane that would not work at all.
Go running, biking, swimming.
Go to a local gym or meet up (running, climbing, activity)
Bring gear - resistance bands, caribiner for handle, cardboard to step on for lower body workout, sling and loops for an upper body row, jump rope for cardio especially if it is rainy inside. just jump during an entire episode or something.