I got an email from Mike Wagner, who wrote:
Have you ever heard James Taylor sing “Secret O’ Life”? I think it is a good
“happiness” song. Perhaps there is an entire category of “happiness songs”
we could gather, or likely someone already has. Simon and Garfunkel’s
“Feelin’ Groovy” might fit too.

I immediately thought of Don’t worry-be happy by Bobby McFerrin. Do you have any additions to the list of happiness songs?