You can find lots of stuff at WDW, holiday and non-holiday stuff.
In the park stores carry more specific stuff - in Tomorrowland you will find small stores that cater towards space travel or Buzz Lightyear and Space Mountain.
Around Liberty Square and Frontierland, you will find pop guns, Crocket hats and Woody type stuff.
Main Street USA has a wide assortment of all types of shirts, jerseys, keychains, hats, towels, etc.
The Chatpeau is a hat-only store that can monogram your hats and Mickey ears. There's a great watch store and bakery on Main Street as well.
EPCOT has Art of Disney and Mouse Gears (two very nice stores). The WS has eclectic souvies from their respective countries (great chocolates in Germany and fabulous silver, paper mache and blanets at Mexico.
The best shopping is DTD. They have a 365 day Christmas Shoppe, ARt of Disney, Scrapbooking and a pet store. The best is World of Disney which is the size of a football field and carries tons and tons of stuff. If you have an AP you can get 10% all your purchases (except art collectibles). That 10% really adds up when you are buying for friends and family.