since there arent too many nouns that you can really use for that purpose that begin with "i" maybe you can somehow integrate the word imagination. perhaps put nothing in the bag, or something that she cant identify and when she tells you "i dont know" you can respond with "use your imagination" and then inform her that imagination begins with "i".

its not too likely that she would know incense by its feel, and iguana is more known as lizard at that age as well as insect is known as bug and infant as baby. if she has a doll with a pair of removable ice skates that would work. if not youll definitely have to be creative on that one.

or if you go with the empty bag and dont like imagination you could go with invisible or just plain old inside since shes feeling the inside of the bag. just a few ideas.