This is ridiculous sometimes what simple things will entertain your toddler to no end! :) I found two caps (I think they are both from different fabric softeners) in my save-for-Kitten-may-be-we-will-come-up-with-something-do-with-it-later drawer (don't laugh, everybody needs one!), which fit into one another perfectly.
And we discovered that if you pull them apart suddenly...They will make a popping sound! What fun! She loved doing it :)