Here are some to get us going,

All in a line with a couple of bikes lengths between each rider, the
rider at the back slaloms to the front, and so on.

Slow race, let the good riders show off and stay in sight. The heros
will have less to prove once they have shown their prowess.

With a mixed group some will be bored on a slow ascent so teach them
to wheelie, or just show you how good they are, even if they are
better than you.

Make a slalom with what you can find.

Mark out a small box and turn the bike around in it.

Make a mini obstacle course.

Track stand competition and coaching.