Just how much you want out of any workout is going to be determined primarily by your mental attitude (goals, persuasion, "want-to") towards your workout. There are plenty of workouts people do not do because they're "hard"--e.g. difficult. There are plenty of workouts people do not do because they're "boring"--no gear/equipment is involved, it's not social enough, it's too "different" (too slow, too long, too repetitive, looks weird, afraid to look odd doing it). I daresay we've all been there when looking for different workouts.
So... as long as you can convince yourself it's a worthwhile workout, as long as you can mentally prepare yourself to WANT IT, you can experience results with almost anything if you put an appropriate amount of effort into it.
Here are some ideas for low-to-no-gear workouts:
- Body weight workouts
- Ruck Sack
- Running (mix it up: do intervals or sprints, try a fartlek, go further/longer than you ever have before, etc.)
- Swimming (don't go it alone; minimally have supervision/lifeguard and a capable swim buddy)
I've linked to some examples for you, and you will find some overlap, but any or a combination of these should provide you with some good options to consider using while on vacation.
You also mention that it's a longer vacation (4 weeks or so). If you will primarily be in one geographic location, you may even be able to find an athletic club or other school (e.g. martial arts, etc.) where you can sign up for a 1-month trial just to try something fun and different.