Never go too simple. Plan a date, picnic in the park perhaps... Get some glow in the dark markers from walmart and write on a big poster "Will you go to prom with me?" decorate it with drawn on flowers and such... possibly some hearts depending on the relationship and hide it behind a tree or behind some woods. Enjoy your date and wait until almost dark. Lead her to it.. very romantic and very sweet
Take her out to a nice restaurant and once you are done take her out to places and when the night almost comes to an end and friends to wear shirts spelling out (enter name here) prom? It may not be simple but if you do it in public, she will live you for it and always remember us girls don't just want simple
Have a whole bunch of people all throughout the day give her things of butter. Then at the end have the office call her down. There waiting for her is a thing of popcorn and it has a note that says "now that I've buttered you up will you go to Prom with me?" then at the bottom of the box have your name there.