I would say the homemade paper cards my two boys would make for me when I was sick. They would make me breakfast, and a card they worked on together and then serve me breakfast in bed. I was a single mom and they started this at an early age and have never grown out of it.
I have Christmas ornaments on my tree that my kids made as children. I also have a block string that says "I love you" hanging from my rearview mirror that my son made me when he was in the 2nd grade. He is 21 now. Also, my kids made me a wooden frame that hangs from a ribbon (it is three frames in one) with them and my granddaughter featured for mother's day about 5 years ago. I love that! Okay... I know that is more than what you asked for but I couldn't pick. Sorry!