Can't go to girlfriend's prom, need ideas?

So I'm 21 and my girlfriend is a senior in high school (18). Her school has a rule that dates who are not current students cannot be over 20 years old, meaning I cannot go to her prom. She didn't exactly say it but I know she was upset about this. I tried to convince her to go without me but she brought up some pretty good points about why she'd rather not. So I told her I'd plan something for us to do that night. My original idea was to have a picnic on the lake front for lunch, then head down to Chicago for a White Sox game (which is also firework night), and end the night with a Ferris Wheel ride at Navy Pier. However, the weather kind of screwed me and it is going to be pretty chilly (high 46 degrees). Am I worrying too much about the weather? Do you guys have any ideas for some nice (read: romantic, not going to fornicate because that will not happen anyways) things I can do for her? Thanks.