I would recommend having more planned than you’ll able to use just in case a certain activity doesn’t work out. Don’t be afraid to calming wrap up an activity if its really not jiving. From my experience, using technology impressed my administrators a lot. I brought my own laptop and used recorded music and segments as necessary.
I teach elementary school so I’m having difficulty coming up with age appropriate ideas but your STOMP idea is great. It could include ostinati, rhythm reading, composition, improvisation, performance, evaluating, and even a quick lesson in concert etiquette! Be sure to think through all the details of handing out/collecting instruments, behavior consequences (also another way to impress administration! Let them know that you can see unexceptable behavior and encourage what you like!) With a small class, it won’t be too difficult to bring your own instruments, just in case!
Let me know how it works! Sounds like a fun opportunity!