Do that 1600 quilt race but change up the design a bit.....add a contrasting square every once in a while, or cut the strips on a diagonal at the join, or add a short (18"?) white strip ever few rows, that sort of thing.
Or you could cut two finished top on a diagonal then sew them back together for a different look.
Maybe put these and other ideas on cards in a bag and have the participants draw one to do.
Sounds like a fun retreat!
Jan in VA