I would say which park you spend the most time in depends on her likes and interests. For an 18 year old young woman, Epcot, esp. the world showcase is probably a sure bet.
Shopping and Snacking around the world, and the performances in the different countries are really cool. I think she'd really enjoy that.
Of course, you'll want to spend some time in MK. What 18 year old doesn't still feel like a little kid at heart? I know when I was 18, I still felt all warm and fuzzy when I saw the castle, and Winnie the Pooh etc. I like Liberty Tree Tavern for dinner as a character meal. I think it's one of the best in the World.
If she likes thrills, MGM is somewhere you'll want to spend more time. Most teens and young adults can not get enough of Tower of Terror and Rock N Roller Coaster. Also, the Beauty and the Beast Show and Fantasmic are not to be missed. She'll love Fantasmic.
If she likes animals (I did at that age, but most of my friends could have cared less), you'll want to thouroughly explore Animal Kingdom, see the Lion King show there and maybe visit Fort Wilderness / Wilderness Lodge to go horseback riding.
Maybe on one of her last days there, you could surprise her with a backstage tour? Keys to the Kingdom is excellent and takes you behind the scenes at the Magic Kingdom. I will be doing the Segway tour at Epcot when I go in May and this tour is open to people over age 16. She might think it's neat to ride a Segway. There is also a Dolphin encounter at Epcots Living Seas. These backstage tours do cost extra $$ but are usually well worth it.
I think that would be an experience she would really enjoy.
Definately hit Downtown Disney for shopping.
As for a spa treatment, I'd say for a first time spa treatment, maybe a facial, manicure and pedicure?