So Long, Farewell
song from The Sound of Music

Listen to this farewell song:

This is the classic goodnight song - cheerful and easy to sing along to.
From the movie The Sound Of Music, which is also a movie classic, it was sung twice - the first time to say goodnight, the second time to say adieu.

Other songs like this one:
For Good from the musical Wicked
This duet, between the characters Elphaba and Glinda in the musical play Wicked, is inspired by the story of the Wizard of Oz.
It is sung at the end of the play, and reflects on how their friendship has changed them "for good", before they part:
"It well may be/That we will never meet again
In this lifetime/So let me say before we part
So much of me/Is made of what I learned from you"

We'll Meet Again by Vera Lynn
Indulge in some wartime nostalgia with this classic song by Vera Lynn.
'We'll Meet Again' is a famous song of the Second World War era, and its optimistic tone a resonant comfort for soldiers and their families and sweethearts who were separated by the war.
It's a perfect song for masking a heartbreak with a smile.

"We'll meet again
Don't know where
Don't know when
But I know we'll meet again
Some sunny day
Keep smiling through
Just like you always do
'Till the blue skies
Drive the dark clouds far away"