Twinkle, Twinkle Little Star

Source: Ed. by Kerby, E. C. Folk Songs North America Sings p. 165

The Lion Sleeps Tonight

Most children know this song from South Africa, performed by musicians such as the Weavers, The Tokens, and the Kingston Trio, from Disney's The Lion King and perform the melody and rhythms easily.  It can be sung simply, or, with the addition of xylophones, recorder, percussion instruments, and dancers can become a show-stopping performance event.
Source: Parker, Lisa Ann.  1993 Evergreen Orff Chapter Workshop Notes
Additional information is available on Wikipedia.

Laugh, Ha, Ha

A delightful four-part round.  I like to use it for an April Fools Day rondo with students supplying knock-knock jokes in between each repeat of the song.
Source: Zanzig, Augustus D.  101 Rounds For Singing Pub. by World Around Songs Inc. p. 17