Basic suggestions for standards are old tunes where there is no copyright to worry about, and besides, everyone should know them already…
- People tend to know the really old folk songs and hymns, because they learned them in school, at camp, in movies, etc. Playing these can help unite. Besides, you can buy these collections in books, pretty cheaply. My piano-playing friend has many of them! Just do a search for “folksong fake book” and you can find plenty of them, songs that all sang together in the old days. Some of them would be suitable. A battle song could be appropriate. You are going into battle, after all, a battle against evil. My choice among those: Battle Hymn of the Republic/John Brown’s Body (some of the lyrics, anyway).
2. You can depend on Irving Berlin songs, snappy tunes, very singable.
The best is God Bless America, and everybody should already know this! And of course, you can add Alexander’s Ragtime Band and other ragtime/jazz tunes.
3. If you happen to have a military or school marching band available, maybe some John Philip Sousa tunes would be nice. My husband was a percussionist in his school marching band, and he knows them all.
I will add a 4th suggestion, though. Old-time fiddle music (and some newer songs) is peppy, makes people tap their feet and get up and dance. I grew up with this. So if you can get a good fiddler at some of your stops, that would be excellent. This should be possible anywhere in French country (NE US border states, Cajun country), as well as Texas, Appalachia, the Ozarks, Nashville, etc. There normally are Bluegrass festivals all over in the summer; should be easy to find amateur players. You don’t need Charlie Daniels (RIP) to have a good time.