For raw beginners, just steering/ keeping leg position/ starting to trot IS interesting, and hard! Don't worry that they will be bored, just give them poles to walk over, cones to weave through, and see how they go! Also practice two point at the walk, and even posting at the walk. Have one camper lead the horse while the one riding it closes her eyes. At the walk, and at the trot if kids are old/ safe enough.
We used to play this great game. I CAN NOT remember the name of it, but was a version of hide and seek. Kid who was 'it' would count to 50 or 100 with eyes closed, other kids would go and hide. Then in intervals 'it' would look for others from base, then close eyes and count again, but a smaller number- like 40, then, 20, then 10. Other kids would try and get back to base, not being seen. Between each count 'it' would open eyes and search for others. Works great if you have a big jumping field or some such-- obviously WITHOUT horses present during game! All kids loved it.
Other games/ activities:
tack parts and cleaning
horse parts- quiz show
older kids decorate old/ donated ribbons, use puffy paint or some such. Use ribbons as prizes at end of week show.
Grazing quiet ponies
They love to jump without horses, and to 'trot' and 'canter'
Horseless rodeo is fun too-- kids are amusing to watch when they try to buck
Have fun! Taught summer camp through college and loved it!