Don't pawn it off as romantic. Cause as Ironliftr3 said, that might be a little wierd. But it might be fun to do it as a 'fun' little thing. I would start off a conversation kinda like, 'Geez. It's too bad that we weren't together before prom. Imagine how fun that would have been.' Then hold a small conversation about it, and if it looks like she does agree that it would have been fun, go ahead and do it if you want, and show her telling her you were bored and remembered that conversation. Then pull out a picture of a monster truck and say, 'Sigh. And look how beautiful our limo would have been too. . . '