Just thought of 2 more:
#1 Sue and I were shopping one day and found these cheap plastic yard flamingos. We bought two and stuck them in the front yard of a co-worker (and friend) who is very meticulous about his home’s appearance, both inside and out. He about died from embarrassment when he got home from a trip, but knew who the guilty party and kept quiet.
A few days later, Sue discovered them in her front yard, now with one donned in a top hat and tie and the other with a wedding veil. A week later, they were in my front yard on Easter morning, along with a birds nest of Easter grass and plastic eggs.
#2 I spent a month in England with my fiancé, and we asked his parents to stay with us as well. At some point, I rearranged the decorative wooden fish and sail boats in the (only) bathroom around and waited for him to say something. The next day, they were also rearranged, but not back in the same place. This went on for a couple of days, to the point where the 3 fish were lined up next to the tub on the floor. We finally broke down and laughed at it in private and decided to wait for his parents to mention it. Nothing. Not being able to stand it any further, we asked them if they even noticed. It turns out that they had, and just thought that we were a bit loony and thus meant for each other.