Hidden Mickeys are partial or complete images of Mickey Mouse or other characters that have been hidden by Disney Imagineers in the designs of WDW attractions, hotels, shops, restaurants, etc. I wrote a book describing about 400 HM's, some of which are already lost since the book was published. Finding HM's is a fun game. Every time I'm at WDW (almost weekly), I find new or lost HM's. Disney has a few HM lists, usually available at certain resorts like Animal Kingdom Lodge and Wilderness Lodge. Ask cast members at any venue in WDW about HM's, and they're usually able to tell you about some that are closeby. For example, the refurbished Mickey's Star Traders shop in the Magic Kingdom has at least four (or more) new classic three-circle HM's in the mural circling the walls of the shop. Have a great trip!
Steve Barrett, author of the Hidden Mickeys book and "The Hassle-Free Walt Disney World Vacation" touring guide book