The "Wing in a Sling" episode was recently added to sesamestreet.org, and I think I found another hidden gem that hasn't been mentioned. At the beginning Big Bird sings a variation of "Row Your Boat", going "Roll, roll, rollerskate, gently down the street, merily merily merily, life is such a treat!" Now, I wonder if that was referencing Don Music's version of the song. The only big difference is the "Rollerskate" part in place of "Drive your car". Hmmmmm.....
Also, I wonder if Kermit's cameo could be considered an honorary hidden gem. Of course there are things preventing that... It was in Elmo's World instead of the street story, it was produced for season 39, each season since Henson sold the rights to the Sesame Street characters (with the exception of season 33) has featured at least one insert with Kermit (I think this is the first in a long time to feature two Kermit appearances), and everybody knows who Kermit is, even if they aren't aware that he was a regular on Sesame Street at one time.