For coordinated group exercise where all participants are completing an exercise movement simultaneously (where you need multiples of lighter dumbbells) – get a heavy duty rolling suitcase or rolling toolbox.
For semi-private training bring a pair of (quickly) adjustable dumbbells; there’s an assortment systems out there. Of course you’ll want to bring other training tools and accessories as well.
I like to have my clients “fight” over my equipment. For example, with 6 clients I pull out 5 tools. They have to race to the car and pick out their choice, or choice that’s left. The last person starts with a body weight exercise (after a group warm up, of course!). This is a lot of fun when training at a park or somewhere outside.