T.A.
This isn't a toy but my DD has a plastic table w/chairs for the backyard. In addition to eating at it, she can do messy arts-and-craft type stuff like painting, play dough, etc. on it. It works out perfectly for us. Plus it would be nice for playdates as well. The kids can sit at their own table. There usualy not too expensive either.
Water play!!! Maybe a sand-and-water type table although you mention that you already have a sandbox. Kiddie sprinklers are fun too. So are kiddie pools.