Sometimes last-minute isn't such a bad thing — I'd planned a series of rhyming clues but was struck with inspiration that morning and instead...
Six mini treasure maps that combined to make the final map!
We had the party at a community center, so there was lots of stuff to put on the map. A backyard could work great, too, though, just with more detail.
I sketched the basic layout and scouted out potential hiding places, then drew the whole map on a huge piece of paper with a permanent marker and a tiny red X to mark where they'd find the next piece of the map. For my own sanity, I numbered the six sections on the back in pencil and wrote where they were supposed to be hidden (because it can get a little confusing). Rumpled the map a good bit to make it look interesting, cut it up in slightly irregular hunks, and put them in ziplock bags because it was a drizzly day.
I did try to do some organization, the first mini-map was easily recognizable as the playground so they wouldn't be too stumped right at the beginning, for instance. (And I'd hidden it in Mr. P's birthday crown so it could be "discovered" when I gave him the crown at the time I deemed appropriate.)
I was worried it would be too easy or too hard, but it turned out really well. They rushed off like mad every time they found a new piece, not paying much attention to which exact tree or which exact lamppost was marked until they'd searched fruitlessly for a while and one of the older kids thought to check the map a little more carefully... "Hey, guys! I think it's that tree!"
They needed a little adult help to realize that the individual maps were forming a big map that would give them the context to identify the things that were a little less obvious on their own, but after that Mr. P became the loyal keeper of the map pieces, which was very sweet.
The last piece of the map was in PaPa's pocket (drawn as a stick figure inside the community center with a red X near the waist), but he had to take it out and conspicuously study it before they figured that out!
The treasure was a box of chocolate coins, pencils, erasers, and temporary tatoos hidden in a big pine tree, which was all alone in it's section of the map so would have been a bit difficult to figure out without the rest of the map, a nice final aha! moment (also, the X was a lot bigger for that one). And the pine tree was bushy and big enough that everyone could be seriously involved in the search. :)
I'm calling this the beginning of a family tradition. Fun and easy to design, fun to watch them figure it out, and they loved it.