Cool project, Greg!
Here is an eclectic group of optimistic songs to add to those already mentioned...
Angels Among Us - Alabama
Put a Little Love in Your Heart - DeShannon or Al Green and Annie Lenox
He Ain't Heavy, He's My Brother - The Hollies
Count on Me - Bruno Mars
You Will Be Found from Dear Evan Hansen. I like this virtual choir project version.
Child Prodigy cover of Heal the World
https://www.youtube.com/watch?v=h6d6Yo3DwVI
A Wink and a Smile - Harry Connick Jr.'s cover from the Sleepless in Seattle soundtrack
Surfin' Safari - Beach Boys
Seasons of Love from Rent
Here Comes the Sun - The Beatles
The Lion Sleeps Tonight - best known version is The Tokens. I also like Straight No Chaser's version.
Eye of the Tiger - Survivor
Don't Stop Believin - Journey
+1 for My Girl - Temptations