Being primarily a mobile business, I tend to use the equipment that travels easily. This may be useful if you’re also a contracted instructor.
I use sandbells and sandbags, as they substitute for a lot of different equipment. Resistance tubes, loop bands, suspension straps, and a couple of heavy kettlebells along with body weight training will give you endless options. Also specialized equipment like cones, hurdles, ladders, sleds, and parachutes are great for athletes or more athletic populations.,
But as others have said, it depends on your participants, but don’t forget the paperwork or administrative/office supplies.