I think just general toddler groups are good at that age, so they have a chance to play with other kids and mill around and get messy. Choose one that does art and lots of messy play so you don't have to do it at home and deal with the clearing up yourself
Local libraries often have good rhyme time/story time groups.
My little girl turned 2 just over a week ago and I wanted to try introducing her to a few specialist-interest groups, however the only one I can think of is Ballet! I just wondered if anyone else could think of anything a bit more original?!