So fun! What a great birthday idea - she will LOVE it!
We do this every time we go to EPCOT and always have a blast. Beware - the cost of this mission is not for the faint of heart - figure anywhere between $6-12 per drink per country.
Some tips (we're rope-droppers, FYI):
~ Knock out as many rides as you plan to do that day BEFORE you start your trek around the World. Especially Mission: Space and Test Track.
~ Start in Mexico - the icy/slushie margaritas from the casita across from the Temple are very light on alcohol, so it's an easy start into the venture. If/when you decide to head back for seconds, head inside the Temple and try a real margarita - or flight of margaritas!
~ If there are a group of you, consider starting the trek by splitting drinks. Keep note of what you like and don't like and where it was - this will come in handy later.
~ You don't necessarily have to drink at the countries in order - if you want to skip, say, France and save your Grey Goose slushie or Champagne until later when you're watching Illuminations, then do it!
~ Look for places to sit and drink versus walking around with drinks. There are great tables with umbrellas by the "Africa"-themed bridge area. Also, there is a great area of tables on the water by Rose & Crown that aren't part of the Rose & Crown. This "research" will come in handy later.
~ Share snacks as you travel the countries. We love to split the slider platter at Tangierine, as well as the udon soup in Japan, nachos in Mexico and pastries in France.
~ Take your time and enjoy the trek. We usually start our trip(s) around the World as soon as WS opens and we go through Illuminations. But we like to meander around, enjoy the scenery (and the nice weather!). We've done treks where we've brought a deck of cards or that Farkle game and have enjoyed playing while we enjoy drinks and snacks at a table by the lagoon.
~ Water, water, water! Even if it's just stopping to sip a little at a fountain, nobody wants to be the first one to call it quits while Drinking Around the World.
We hardly ever do TS at EPCOT because we snack around the World. About ~45-60 minutes before Illuminations, we have a pretty good idea of what drinks we want to close our night out with and find a table close to those locations. For instance, my husband really enjoys fancy beers, while I like champagne/prosecco to end my night. If you've been keeping track of what you like (maybe in a Passport?), you can find a nice table along the water so you're close enough for you nightcap drinks!
As far as the drinks go, other than the Grey Goose slushies, I don't think there many are of a high enough ABV that would really mess up a newly-minted 21 year old while drinking around the World. Encourage the slushie/mixed drinks over wine/beer unless they have experience with that. But definitely have at least one glass of Champagne/Prosecco to celebrate the birthday.
Also, coconut water and Emergen-C the next morning are a quick fix for any potential hangover!
Cheers and enjoy!!