I think the first thing is what does she like to do? Does she like going to the movies, dinner, etc? Also, is money an issue when it comes to doing things for her, or is it simply you do not know what to do?

i would usually start off the day with brunch at her favorite restaurant. afterwards, I'll drive her to a spa and pay for it (if you have the money pay for a few of her friends as well). while she's there, take the time to buy a bouquet of flowers, get her a decent bday card and put some thought into it. always make sure to tell her how pretty she looks. after the spa hang out with her and her friends for some brownie points. leave it up to them to decide what to do. end the night by watching a romantic movie with her.

it all depends on what the two of you enjoy doing and what attractions are offered in the city you live in.

Depends 1. from her interests, 2. time you are together 3. amount of money 4. your age 5. stuff that you can do in your town or near....

For me ideal would be, at the moment - day off, no college, no job, going to long walk in park or somewhere outside of town, then lunch, then maybe going back home, taking a nap + cuddling, then having friends on drink or two, and then evening/night together.
Or, instead of walking or hiking it can be evening at theater or movies or at some really good concert.

And some small present, of course, like tickets for show or concert, or a book I would like to have or reservation for weekend in youth hostel next month - something like that.