Hi Rich
I dont run a hand class as such but teach Clinical Pilates as part of being a physio. I got interested reading your "problem" - (I'm a new mum and ran a couple of classes with new mums and I know what we can be like!).
Anyway, not sure what functional level your group is at, but wondering if you have tried the following:
UL weight-bearing exercises: eg wall pushups, kneeling on fours, - gives a more "challenging" objective like a pushup and makes them feel like they have "exercised"
Sitting on Swiss Ball (SB) and pulling on Theraband's (TB) - try sitting forwards on SB with feet and knees together on ground and pulling on TB. As a further challenge, get them to lift their heels off the ground while holding onto tension in the TB.
Kneading dough, making cookies, making pasta etc etc for strengthening fingers. Knitting, sewing etc for their kids or to sell to fund raise for charity.
Good luck!
Elin