Hey -
For an 18 month old, The Little Gym is a great place. They have a "Mom and me" class for about that age. It does a great job of exposing them to music and to "rules" in that there are "starts" and "Stops" to the activities. Further, it works on the gross motor skills that a child that age needs. My son had sensory integration disorder and was actually in therapy (occupational and physical) at that age and they recommended the Little Gym class for him. He has since discontinued therapy, but still goes to gymnastics (he is 4 now). I was AMAZED at how much he enjoyed the gross motor skill activities and how much he learned from them.
As for art classes, I wouldn't really consider those until he is a little bit older. My son took an art class (summer class for 2 weeks) when he was 3 and LOVED it. But before that, he had not really been interested and did not really have the fine motor skills to enjoy doing it. Now, he draws all the time.