I forget exactly when, but some BC kid suggested we trick or treat on Halloween. I offered the goofy suggestion of hiding candy in the lockers, making them bring candy buckets, and going door to door.

What we actually did was pretty goofy.

I used 8 lockers, and inside was either a trick or a treat.

I constructed a little puzzle. I borrowed some locks from athletics which have serial numbers. Bulk combination locks have the serial number and a key slot on the back so you can recover the combination or manually open the lock if a kid forgets the way in (or if a random lock appears). The task would have two parts: decode the combination, and find the lock it corresponded to.

For the clues, I had this idea that the lock would be "worth its weight in gold." The serial number would represent a weight, in grams, of gold. If you translate that into moles you get a cryptic looking number. I used that molar count of gold as the envelope label. Inside were three problems, either two derivatives and an integral or two integrals and a derivative. Inside was also a clue mentioning that "two like expressions wouldn't be caught dead next to one another." The intent was kids would realize the combination went D I D or I D I. Evaluating the three problems would yield three numbers that opened the lock in some order. Multiplying the number on the envelope by the g/mol of gold would convert it to the serial number.

I sent out some cryptic hints on our class Remind.

It worked pretty well. They were really confused/interested as to what the heck might be happening. On Halloween, I read some brief instructions, turned off the lights, and they went scrambling. They picked a clue at random and a partner. Five lockers had candy, three did not, but they'd have no idea until cracked the puzzle.

Admittedly I was a little too clever. They figured out the numbers were the combination. Translating the number on the envelope into a serial number had them chasing a lot of random ideas and eventually I had to offer some hints. Most chose to brute force the combination (trying the numbers in various orders until it worked) rather than think about the clue about like expressions. One kid wanted to try all possible versions of his combination on all the locks, a process he didn't realize would take forever.

Overall it worked. The idea just needs a little more refinement. The lock opening phase was particularly intense once they realized it might have all been for nothing.

I think there's a version here that scales for AB.