Don’t spend more than you can afford.
Sure, every woman secretly dreams of getting the Pretty Woman treatment from their Richard Gere (you know, minus the whole prostitute thing). But if you don't have that kind of cash, don't feel compelled to spend like you do.
The inclination to go overboard, while born of a sweet sentiment, is not necessary. Your girlfriend knows your cash flow, and she doesn't expect you to make it rain money you don't have just because it's a holiday.
Not only will the gesture make you look like you're trying too hard—willing to go into debt to make her happy? Not a good look—it will also make her feel guilty. Spend what you can afford, and make sure what you get is thoughtful. That's all any woman can hope for.