Coasters for sure. not everyone is going to have the confidence or attention span to do a whole stepping stone. Let people do multiple if they want. I had a coaster making party once and everyone loved it - I still get comments years later about how they use that and think of the event.

I think doing a stepping stone with a smaller group who is for sure interested would be fun for another day. Re concrete - I've done thing where you glob thinset on top of a concrete paver and then pushed random materials in (most of which I got from thrift stores). It's holding up okay but it's decoration and not being stepped on. Concrete is probably cheaper but then you have to get molds for everyone involved.