My daughter celebrated her 5th birthday in the park this past week. We bought 60th celebration cupcakes (which are BEAUTIFUL and delicious) for everybody at Fiddler, Fifer, and Practical and sang her happy birthday. We booked her an appointment at the Bibbity Bobbity Boutique in the morning, which she LOVED. We also took all of our kids to see "Inside Out" at the AMC in DTD, which was a super fun birthday treat outside of the park. I know that's not applicable for your son's 18th birthday, but I'm just saying, we found ways to make the trip special and to celebrate her birthday without buying the expensive celebration package.
Definitely stop by City Hall and get him a birthday button. Consider making reservations at a restaurant you normally wouldn't eat at (such as Blue Bayou). The 60th diamond cupcakes are beautiful and make for a great birthday treat.
@JessieD - You can buy Disney decorations at Party City and decorate the room while he sleeps. They have all kinds of Disney character party decorations to choose from. Waking up to that would be amazing! Also, if he's into Star Wars, DEFINITELY take him to the Jedi Training Academy. Get there about 30-40 minutes early, sit in the very front of the group (wearing Star Wars attire helps), and when they ask for volunteers, have him be as energetic as possible to be chosen. That would be an amazing birthday memory. A character meal could also be neat. We took our son to Surf's Up with Mickey, and he was THRILLED. It was such a lovely, laid-back experience, the food was wonderful, the characters are so interactive and silly, and we had a great time.
Other birthday or celebration ideas:
-The Build a Bear in DTD is fun for little ones. We had never been before, so we took our 3 year old (each kid got something special one on one with me and DH- DD got BBB, older DS got the breakfast with Mickey, and youngest DS got Build a Bear). He loved it. It was so cute.
-Bibbity Bobbity Boutique (boys can be made into knights, girls into princesses)
-Special character meet (we took DD to meet Anna and Elsa, which requires a FP)
-Character meal
-Meal at House of Blues
-Meal at Blue Bayou
-Call from Mickey (if staying on grounds)
-Decorating room
-A birthday hat or class of 2015 hat (sold at Mad Hatters)
-Movie in DTD
-Brunch in DTD