So there's a group of musicians who play as needed / as available, and occasionally we sing unaccompanied. We need to grow the pool. Someone has offered to run group guitar classes for kids - the classes are cheap, but they have to play (something) at Mass once a month. The catch is that the teacher hasn't been involved with church music for 30 years, so he's not sure what hymns might be suitable anymore

So I'm looking for suggestions. Simple meditation / reflection pieces they could use at offertory or communion, most probably (reduces the risk of clapping afterwards). But open to other ideas too.
Our current repertoire is mainly the sort of music that makes most people on this list despair! Simple, very well known and singable, with lots of repetition. But we do sing Mass parts - and I'd love to make sure there's at least one or two on the list, so they get the idea of singing-the-Mass too. (We introduced the Glendalough Holy Holy this morning - went ok, but the tune is too complex for this group initially - and there are no guitar chords in the published version we have.)
Any ideas?