htmlwidget for R to sneak Easter eggs in your Rmarkdown documents: egg.js.
You can install egg from github with:
# install.packages("devtools")
devtools::install_github("JohnCoene/egg")
When the user strikes the keys "type" in your document the egg will appear.
egg(c("t", "y", "p", "e"), "EASTER EGG", imegg = "https://upload.wikimedia.org/wikipedia/commons/5/54/Bg-easter-eggs.jpg")