A Giant Gift
Many couples, like many friends, have a common symbol between them, something that reminds them of each other every time they see those symbols. A great idea in that case is to draw that symbol on a large scale, for example, in a field (by joining stones) or on the beach (leaving the mark in the sand with your foot) so that the symbol can be seen from the window of a building.
Make sure the surprise is huge, you just have to tell that special person to close their eyes and as soon as they're in front of the window, 'Surprise!' It will be something hard to forget.
Finally, if you don't have a house that faces the beach or a park, you can always ask your friends, surely one of them will have one. If they are in another city, they can always draw that symbol and take a picture of it from their home. After all, a picture is worth a thousand words.