Pretend to fall off a bridge while proposing. Then fall off the bridge for real.
In all seriousness, stage a cheating session with a female friend or hired actress. Make your girlfriend come home and catch you in the act with another woman (except it's hilariously faked). Then propose to her. The combination of heartbreaking disappointment and triumphant joy when she realizes what's actually going on will make for an amazing moment. There's nothing like combining a wonderful surprise with prior disappointment to make the surprise that much more effective.
In all seriousness seriousness, though, it really doesn't matter what you do and where, as long as it's special for both of you. If you try to stage something, it will forever have been just that: staged. More to the point, if you do anything out of the ordinary, you broadcast what you're about to do. I can't count how many girls I've known who go "So and so is taking me to X place, I think he's going to propose".
BORING!
ANTICLIMACTIC!
But ultimately, if you ask others for advice on something like this, it will forever be someone else's idea. Find something that touches you two, and do it.
Myself, one night after we decided we couldn't be apart much longer, I drove an hour and a half to my girlfriend's home at 3 AM and climbed up to her window to propose to her with a cheap plastic ring I'd purchased from Meier. She was awestruck. Not planned, but very, very special.