Rather than written thank-you cards, I had the four-year old give a taped video statement of gratitude to each of his gift-givers who were present at his party (the videos were emailed to each individual).
For some reason, watching the whole sequence of videos gave the Missus one of the best laughs.
The earnestness of the birthday boy is appreciated, but we came to enjoy the gradual slumping of Sidekick Simon as he experiences the challenges of gravity.
Eventually he did topple over, and had to be replaced by big brother.