What about layering it with a delicious fruit curd? You could do traditional lemon or go for something a bit different like grapefruit or lime.

I love using edible nasturtium flowers on cakes/cupcakes - you can even 'sugar' them to add another level of delicate appeal. I can usually find them in the packaged herb area of my grocery's produce section.

Does she like chocolate? How about some chocolate drambuie mousse between the layers? (you could use any liqueur she likes, in place of drambuie - makes it a bit festive!)
http://outoftheordinaryfood.com/2012/02/16/chocolate-drambuie-mousse/

(I assume you can leave it in the fridge?)

I know it's not vanilla, but whenever I have to make something for a GF birthday, I use the Bob's chocolate cake mix with this doctored recipe http://www.thedailygreen.com/healthy-eating/blogs/gluten-free/gluten-free-chocolate-cupcakes. I add a bag of semi sweet chocolate chips to the batter before baking, and top the finished cupcakes with home made peanut butter frosting. I refrigerate them over the night and the next day they are a very decadent fudgy-peanut butter treat. People who don't know never guess they are gluten free.

Be careful about adds in if she is g-f. Funfetti especially may not be g-f. I agree with the chocolate and fruit ideas. You can pour a chocolate glaze on top and ring fresh fruit around the top or completely cover the top with fruit. A whipped ganache is good and holds up well but it may be better as a filling if it has to keep as it may discolor in air overnight.

If you're sticking with vanilla, you could go extra fancy and do a riff off a Cipriani style Vanilla Cream Meringue Cake. As long as the restaurant can keep the cake chilled, the meringue and pastry cream should hold up just fine. Otherwise, I'm always a fan of layers of fresh fruit in between the cake... Maybe use dental floss to slice your layers into even thinner ones, use strawberries in between, and cover with a fluffy buttercream and some dark chocolate shavings? Also, you could use a piping bag to frost your cake, rather than just an offset spatula. Maybe something like this? http://www.marthastewart.com/sites/files/marthastewart.com/images/content/pub/baby/2003Q2/kids_rufflecake_hd.jpg

I've used these recipes before with great success:
Buttercream: http://www.mangiaphilomena.com/blog/2012/10/29/no-one-eats-buttercream-because-theyre-on-a-diet.html

Meringue Cake: http://www.marthastewart.com/343484/meringue-frosted-cake-with-raspberry-fil?center=276954&gallery=274212&slide=260674

Thanks everyone! I ended up adding poppy seeds, cardamom, and almond flavoring to the batter and am going to make a very very vanilla-beany buttercream to frost it, and top with lots more poppy seeds.

Wow and yum. And happy birthday to your girlfriend!
How did it work with the restaurant?