Balloon Surprise
Idea sent by Mayra.
The idea I propose involves getting a large box and placing a small gift or token at the bottom (whether it's for an anniversary, birthday, etc.).
Then, fill the box with helium balloons, close it, and place the box either at the entrance of the house, next to their vehicle, or somewhere the person you want to surprise will definitely pass by. Outside the box, place a note indicating that the box is for that special someone and that they should open it.
The exciting part is seeing the person's face when they open the box, the balloons fly out, and they find the gift you prepared inside.