Actually, David Cassidy on Glee would be a terrific idea. They could do a whole late 60′s/early 70′s musical revue:
“I Think I Love You” (The Partridge Family)
“Day Dream Believer” (The Monkees)
“Tra La La” (The Banana Splits)
“Best Friend” (The Courtship of Eddie’s Father)
“Suicide is Painless” (MASH)
Jesus, I’m old.