A sneaky leprechaun sneaks in and steals the Lucky Charms while the kids are at lunch! He leaves a note in it's place that leads them on a fun little hunt all over the school! The treats are eventually found back in the classroom. I usually do a sorting/graphing of the marshmallows after the hunt.
**I tried to make the clues/locations as general as possible, so they could be used with any school. :)