Quick and dirty, for use in a pinch
Wizard level = d6 + average character level
Spell DC = 12 + d4 + spell level
Roll on your favorite random name table. The wizard has one spell left for each letter in their name. Start with the highest level spell they can cast - is there a spell that starts with that letter? They know it. Is there no spell that starts with that letter? Go one level down. Repeat.
UNCRETTIN, level 10 wizard
Unseen Servant. Nightmare. Cloudkill. Ray of Exhaustion. Teleport. Telepathic Bond. Enervation. Ice Storm. and Nondetection.