Suggest a birthday walk around the block where you've planted many a surprise for your unsuspecting loved one.
Scope out the area first and come up with places where you can hide a poem on paper, place a flower in a mailbox, or stash a trinket under a rock.
Get your neighbors in on the plan if possible. Maybe your first stop can be next door where you grab a quick drink.
Plant your surprises when the time is right. About 5 items are plenty.
Take your walk and lead the birthday girl or guy to the surprises with simple phrases like, "I wonder what's under that rock" or "Let's check the neighbor's mail." A bit corny, I know, but very sweet!