I like the idea you have. Fill it with balloons too so when she opens the door they all fly out

But other things you can do that's cheap is make dinner together afterwords. All you'd need is the ingredients. Then after you guys eat and stuff you could always watch a movie together. It's simple and cheap.

If you want to be more romantic, make a cheapish dinner yourself, and then set up a table. Be all corny and act like a waiter. Have a towel on your arm and serve it to her like she's in a restaurant. Bow when you give it to her, then show up as her date in completely different clothes. Then afterwords tell her you got movie tickets. After dinner, go into the living room and have it set up like it's a movie theater. Have a box of popcorn (or bowl) ready, a soda with straws and candy and stuff.

Just some ideas. It's not only the gift, but the experience that you have together that make the day special.

I agree with Trillium I think you might need to add a little bit more to it. I think maybe still get her car detailed but blind fold her before you take her to look at the car then when she opens her eyes she sees what you have done for her as it ads a bit more suprise...then when she thinks that's all you have done (although thrilled)... maybe inside the car (part 2) is to have a pre-organised picnic with yummy food ready to go somewhere. So if you're not good at cooking this also helps. The teddy could be in the basket too. Maybe leave love notes in hidden places in the car too so that she finds them months later when she's trying to find things in the car. Or do the love notes hidden in places in her bedroom. It's so nice finding surprises like that & makes you want to tear your room apart trying to find the rest. Just make sure you hide some really well so she wont find them for at least a few weeks.

Hope this helps

All of the above ideas +your penis and you win

pizza and a movie or something, go to a party, etc...