I don't know how romantic it would be but we did a special surprise for our future daughter-in-law through Gifts of a Lifetime. They have a website and many different ideas. My only regret was that my daughter-in-law told EVERYONE about it afterwards so I can't hardly do it for anyone else now!!! I know they've done things for marriage proposals so I'm sure they have some romantic ideas up their sleeves.
Also, dinner at Victoria and Alberts is very romantic. The place is small, probably only about a dozen tables, no children, excellent food. It feels very private. There are only two seatings each night for dinner so it is very relaxed and unhurried.