Hi Alyssa,
What a great way to celebrate a Birthday!
Be sure to pick up a Birthday button for the person celebrating their birthday from Guest Relations as you enter the park.
You can absolutely order a cake to be brought out during your meal at the
San Angel Inn. Specialty celebration cakes are available for an additional charge and
should be ordered 48 hours in advance by calling (407) 827-2253.
As for other ideas to make an adults birthday special, as you will be spending the day at Epcot, why not set them a little challenge and see if they can manage to have an alcoholic drink from each of the countries in the World Showcase. My partner loves to do this during each of our trips just for fun.
Or, perhaps you could try one of the Tours & Experiences on offer at this park. You can read more about all of the different offerings
HERE.
Wishing you a magical trip!