He printed out the code and ripped it into three sections. He sent one to each niece by mail with a note telling them to bring it to the birthday party. Jorge got the third piece. Katie, meanwhile, got a coded message which, with Jorge's help, she could partially solve that morning.
Once at Grandma's the kids fell to solving the code.
It yielded a riddle directing them to the map. It took a minute to figure out, but once they got it they tore off for the basement where a treasure map (drawn by Rob on a scrunched/burnt brown paper bag and marked with Pirate-ese like "Kid's Landing," "Grandma's Fort", and "Here be Geese!") was hidden under the stairs.
They also found a picture of an outdoor fireplace taken in one of those extreme close-up/strange-angle ways that it takes a few minutes to figure out what you're looking at. At the fireplace they found a foil-wrapped jar lid containing another picture. They marked a "1" on the map and ran off to find #2. They marked their "2" and used that picture to find #3 and so on.
Rob, brilliantly, made them run all over the property a dozen times.
The treasure chest (which Rob made out of some cardboard boxes) was filled with chocolate coins, candy necklaces, rolos (aka gold pieces), and various chewy candies (gems).
Rob's mom made a light dinner and his sister made a cake so the rest of the day consisted of typical birthday fun and gifts. Katie and Jorge were both bubbling over with excitement when they told me about it. Katie's only complaint: it should have been harder!