The Surprise Banner
Is it your partner's birthday? Are you celebrating the anniversary of an important date? Do you have something special to celebrate? Announce it, let everyone know so they can help you make the day in question absolutely unforgettable.
A variation on the banner idea is to use flyers instead. In this case, instead of printing and hanging banners, we'll distribute flyers to as many people as we can, either by handing them out manually, leaving piles in various strategic places, or tossing them into the air. The idea, as before, is to reach as many people as possible to help us congratulate.
At the end of the day, don't forget to take down the banners. If you think this will be too much work, you can always indicate the celebration date on the banners to avoid confusion after the said date.
A final warning: consider before carrying out the idea if the degree of shyness of the person you want to cheer up is low enough for them to enjoy the whole 'show' you're going to prepare. Remember, it's about adding a magical touch to a special day and not causing anyone great embarrassment.