“D” is always safe!
For beginners:
The open D string will always fit in the chord. Have your class play the D string in whatever rhythm they’d like! It could be long notes, familiar rhythms, sul pont, loud, quiet, whatever they want! The point is just to make a noise that isn’t from a page. Play along with them for encouragement; and after they get the hang of it, offer to let enthusiastic individuals or small groups take a solo for a few seconds.
For intermediate players:
Sure, the open D string always fits, but why stop there? Try any note from the D major scale, walking up and down (or, if you’re feeling adventurous, skipping). If you hit a note that doesn’t sound quite right, no worries - just move to the next note up or down, and it’ll probably sound great.
Have your students take some familiar tunes they’ve played before and see if they’ll fit over the chord changes. If they don’t fit, challenge them to change it up so they do. Still doesn’t fit? No problem, tell them to pick a different tune they remember or even come up with their own! The only catch is: no notation in front of them.