I get so tired of the usual vertical arrangements, that I always find contrapuntal pieces a respite. Some of my favorites (which aren't too difficult, nor should the music be too hard to find):

Noe, noe

Psallite unigenito (Michael Praetorius, 1609)

Allons, gay, gay, gay bergères (Guillaume Costeley)

Sherburne ("While shepherds watched"; text: Nahum Tate, 1696; music: Daniel Read, 1783) - a spirited shape note version with a fugal section.