Best Things to Do for Your Girlfriend On Her Birthday

I think that's a bit too early.

She is more likely to say yes

Even if you're voice isn't flawless, (mine is actually really good but NOT flawless) singing a girl a song, will either make her love you even more (if you're already in a relationship) or make her love you, or AT LEAST make her think you're sweet! Done it before trust me! (I wrote my own song about/for her in specific, but I'm sure if it's an existing song it will work too! )
Some love songs I think would be perfect for a girl are:
-Open Arms by Journey
-Faithfully by Journey
-Send Her My Love by Journey
-Good Morning Girl by Journey (Short and simple song)
-Stay Awhile by Journey
-Oh Sherrie by Steve Perry (ONLY if girl's name is two syllables then change "Sherrie" to her name)

How romantic!

That birthday gift is very, very expensive.