Do you and your girlfriend love seeing live music? If so, whether you like punk, classical, country, rock or any other style of music, why not propose at a concert?
Take a look at the schedule for your favorite clubs/venues. Find a night that a band that she likes or that both of you like is playing.
Make sure that she's able to go to the show, buy tickets and keep the date clear on your calendar.
Make a plan. If you know someone in the band, see about getting in touch with him/her to see if they would be willing to make the proposal from the stage. If you don't, talk to the managers at the club - or the band's management - to see if they could help you get the band to participate in the proposal.
Once you know that the band is on board, plan the song dedication that you'd love them to give and get it to them.
Buy an engagement ring so that it's available when the band delivers the dedication.
Go to the show and be sure that you're ready when the dedication is made.