My school did Ruchnius vs Gashmiyus one year— it was very entertaining, to say the least! (That was grades 1-6, with help from teachers.)
All ones I’ve seen:
Shemesh vs Yareiach
Shamayim vs Aretz
Ketchup vs Mustard
Pesach vs Shavuos (we did Chanukah vs Purim one year, but that obviously wouln’t work)
Torah vs Avodah vs Gemilus Chasadim