Premium AP gets 15% off for dining, 10% off in stores (minimum $10 purchase).
Plus free parking.
Typically, I ask "do you give an AP discount" and whip out my AP. More often than not, the answer is YES!
Make sure to ask at the stores and dining options in Downtown Disney, many do give AP discount but it may only be certain days (mid week only, no weekends) or before 5pm for example.
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From Mousesavers web site
Annual Passholder Discounts
Disneyland Annual Passholders get nice merchandise discounts at the Disneyland Resort. A single $10 transaction required before tax. Not valid in combination with any other discounts or promotions or on purchases of original and limited-edition artwork or selected collectibles. Excludes the following locations: Names Unraveled, Jewel of Orleans, Silhouette Artist, Portrait artists in New Orleans, Parasol cart, Heraldry shop, Crystal shops on Main Street and in New Orleans and Blast cart.
Premium Annual Passport holders get 10% off merchandise at most shops in Disneyland, Disney's California Adventure and Disney hotels as well as Downtown Disney shops, including World of Disney and Marceline's Confectionery.
Southern California Select Passport, Southern California Annual Passport and Deluxe Annual Passport holders get 10% off merchandise at most Downtown Disney shops, except World of Disney and Marceline's Confectionery.
Must show valid Annual Passport to receive Passholder benefits and/or discounts. An Annual Passholder must present his/her valid Annual Passport prior to purchases to receive any applicable benefits and discounts. Benefits and discounts are nontransferable. Subject to change without notice. Thanks to Darkbeer for additional info.
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Disneyland Annual Passholders get meal discounts at Disneyland Resort. Unless otherwise specified, discounts apply to the Passholder and the Passholder's dining party. Not valid at outdoor cart locations, snack locations, Club 33, Napa Rose, and Room Service locations. Dining discounts are not valid in combination with any other discounts or promotions and exclude alcoholic beverages, tax and gratuity.
Southern California Select Passport, Southern California Annual Passport and Deluxe Annual Passport holders get 10% off meals at virtually all dining locations, including most Disneyland, Disney's California Adventure and Disney hotel restaurants, as well as Downtown Disney restaurants.
Premium Annual Passholders get 15% off meals at virtually all dining locations in Disneyland, Disney's California Adventure and Disney hotels, and 10% off at most Downtown Disney restaurants.
http://www.mousesavers.com/dlmeals.html#annual
http://www.mousesavers.com/otherdl.html