Reddit reviews The Disneyland® Encyclopedia: The Unofficial, Unauthorized, and Unprecedented History of Every Land, Attraction, Restaurant, Shop, and Major Event in the Original Magic Kingdom®

Here are my favorites:

Unofficial Disneyland Encyclopedia

Mouse Tales

The Disney Mountains

I agree with /u/mildly_interesting and the suggestion for the Bingo Cards. I've used those once before and they're pretty fun!

If you like puzzles you should consider a "MouseAdventure Vacation" packet. They collect a handful of "Quests" from old events and put them into a bound booklet you can take to the park with you. It's not everyone's cup of tea, and some of the puzzles are tough. But it can be fun as a group to solve. Check it out here. (The packets are available a bit down that page.)

Lastly, I'd suggest picking up a copy of this book. The Disneyland Encyclopedia. While not a treasure hunt, it has SO much information on Disneyland that you're bound to find secrets and facts about the park that'd you would have never known. On top of that, you can go find those previously unknown details in the park and see them for yourself.

Something like this?

https://www.amazon.com/Disneyland®-Encyclopedia-Unofficial-Unauthorized-Unprecedented/dp/1595800689