at the playgroup i go to now (and the one we went to in sydney) we had a roster for craft time (and story time and morning teas) and each week a different mom organized the craft (idea, supplies and clean up) - it woudl range from simple to more involved depending on the mom and also on the timing (if close to xmas usualy do ornaments, if around easter do soemthing related).
we went to ours yesterday and it is a huge playgroup for us (estimate 25+ kids ranging in age from 13mo - 4 1/2yo, whereas in past used to average of 10-15). the craft yesterday was super simple: in music and story time we had sung "we're going on a bear hunt" and had a bear story so the craft was a photocopy of a teddy bear and the kids decorated them - toddlers had crayons and stampers (hand held paints with ball at the end / like for bingo) and the older kids (2yo+) had access to paste and fabric remnants to decorate with. they had a ball and it was cheap, easy and little mess. at home we do the paper plate decorating thing (with craft feathers, foam shapes, jewels, pom poms) and the girls get creative in their own way. can also recycle stuff from home and have everyone bring in toilet paper (and paper towel) tubes to use in crafts, egg cartons, cereal boxes etc all make good bases for creating things and painting etc.
there are a ton of sites with craft ideas and recipes for things like finger paints... and you can find age appropriate ones, inexpensive ones, simple ideas etc to suit your group. they average only 15 min for craft session at playgroup (often less) so i say talk to your GF and give it a go. if the other moms are intimidated, maybe you guys do the first 2-4 times and then others will take on a role. i am not crafty but i sign up for 1 craft turn each term and hope for the best (even my disaster xmas ornaments one was still a hit with the kids - they don't know or care what it is SUPPOSED to be, like Kate said just let them at the supplies with SOME direction)