It is so wonderful that you are trying to make this a special event for the girl you are interested in! I found a few ideas that I thought were really nice:
1. Spell out the word "prom" in post-it notes on the family's garage door (ask permission from her parents first). On the ground, lay out flowers in the shape of a heart. Wait close by with a bouquet so you are ready when she pulls in the driveway!
2. Does she have a pet? You can attach a little sign to the pet with the word "Prom?" written on it.
3. Go over to hang out and order a pizza. Ask the restaurant to spell out the word "Prom?" in pepperoni. Have her open the box when it comes.
4. Grab some sidewalk chalk and write out a special message to your girl on the street or in the driveway.