I've been singing a lot of harmonies and playing various instruments on many of your choices. I wanted to add some recorder to Cleod's Greensleeves (alto or tenor depending on key).
If I may lead one now, I'll use guitar for one of my recent favorites, Early by Greg Brown. We'll do the chorus twice each time through so that everybody can learn it and join in with lots of harmonies.
Then (since I've held off so long before joining in) I'd like to go a cappella for a short one in unison from the Shaker tradition. Let's do Love Is Little.