Today, with a rowdy, holiday-weekend Jr. Primary, I decided at the last minute to do the good ol' hide-the-magnet game instead of my planned activity. They LOVED it - Jr. and Sr. both - they sang great and participate the whole time! It's a game they played often with the previous chorister.
Basic idea - you pick two children. One goes out into the hall while the other hides the magnet (a good-size clamp one, like the kind you use to seal food bags or hang notes on the fridge with). When the hall kid comes back in, the primary sings louder/softer depending on how close the hall kid is getting to the magnet.
Super simple. No prep. And, apparently, a fave for our primary.