I'd choose Dave's suggestion but I haven't got a romantic bone in my body.
loads of good ideas here but I would suggest that you think what would your girlfriend find romantic. Some of us would love everyone else being in on it - others would prefer a 1-2-1 moment.
My son took his girlfriend to a restaurant in
Marmaris on the beach - think it began with a H - havana or Hana or something like that. He booked the table nearest the sea. They'd already got engaged but he asked her again so she'd had the everyone around and then the quiet moment. The young romantic (can't believe he's my son!) had their fav song on CD which he'd given to the waiter - when that played, he got her up to dance (thought it's not a dancing place) held her tight and said it all again. She said she melted. Think I've just found a romantic bone!
Whatever you do - it's about the two of you and no-one else so make it special for both of you. You're as important in this as her. But enjoy it all.