First, I want to say that I set the playdough out again for the toddler art group this week without any tools. And as far as I could tell, it was completely ignored except for the brief period when I and another mom poked and squeezed it ourselves and vainly tried to encourage our offspring to do the same. It was on the toddler table in the area where we spent much of our time hanging out before and after going back to the studio for our "official" art activity (fingerpainting, which I'll post about soon). If anyone has any tips for getting the toddlers to explore the playdough with their hands, I'd love to hear them!
I did get a lot of great ideas for tools and decorations from the comments that you left, though. Thank you! Zoe from Playing by the Book mentioned that her girls liked to decorate playdough with feathers and googly eyes, and I knew Maia would love to do the same.
Yesterday I got out our feathers, googly eyes, and the playdough for Maia to play with after Daphne went to bed. She had a blast and made a friendly little monster family with them.
The next morning Daphne found the playdough monsters (we left the playdough out overnight — oops!) and was fascinated.
She poked the rest of the googly eyes in the playdough while I did some laundry.
I'm going to continue to present the playdough without tools sometimes, but we enjoy tools (and decorations) too much to do without them completely. And with an older sibling in the house, I can't control Daphne's environment and experiences quite as much. In fact, I'm already planning to try some of the other ideas you suggested — rubber stamps, shells, beads, hardware, clothespins, corks, using koolaid or watercolors to dye the playdough, etc…