Two come to mind (both have ulterior motives):

"I'm lost - or am I?" If you have a mixture of experienced and inexperienced hikers, and a stretch of trail that forks, stop about a quarter mile from the fork. Send two experienced hikers out, tell each to go about a tenth of a mile down each fork, and stop the inexperienced ones as they arrive. Then send the inexperienced hikers down the trail, with enough information (and maybe a simple, non-topo trail map) to be able to figure out which fork to take. (Don't say, "take the left fork" though.) Send them, one by one (about 5 minutes or so apart) down the trail. If they take the wrong trail, your "catcher" stops them and sends them back. The "catcher" on the right trail stops everyone, moves them off the trail, and eventually the group gets back together. (You come down the trail last.) The point is, everyone gets to hike alone, and make a critical decision that they then have to live with for awhile. (Most have never had to do this.) They should get a taste of that feeling we all have, when we're not 100% sure we've gone the right way, just on the verge of maybe being lost. Better they get it in a controlled situation, than the first solo trip they take.

Scavenger hunt: prepare lists before you leave, give one to each hiker, and see who can find the most stuff. Suggestions: something green that isn't grass, something hard that isn't a rock, something wet that isn't water, something that used to be part of an animal (decide whether scat counts), etc.