Making proposal is a very memorable moment for both of a loving couple. That's why it should not be a simple question in a cafe.
I would like to picture for you as I see the proposal. I think my idea is very romantic!
So, invite your girlfriend to take a walk. Drive her to some picturesque place, may be a wild nature plain or a field with flowers! It's up to you! At the moment, you will be walking along that wonderful scenery your girl will see a big air balloon. If she won't, pay your attention to it! The point is it's not the simple air balloon, your friends should hire it at your request. When they see you, they should unroll a prepared "poster". It may be a big sheet of fabric that reads something like "Mary Simpson, please, marry me. Brian". When your gf'll see it, your friends should additionally drop great many flowers from that air balloon. So, I am sure this sight will be fascinating and won't let her indifferent. That day you will get her affirmative answer!!!