Ice Cube Sorting Tray

An ice cube tray makes for such a nice little sorter for toddlers.  I have a packet of colored paper shapes that I inherited from a preschool teacher, but it seems like it would also work well with colored pom poms.  At first, I tried to encourage G to sort according to color or shape, but as this led to frustration, I backed off and let her follow her own little system. 

Any other ideas for things that might work well to sort in the ice cube tray (but that aren't choking hazards?)