Give them a landed or honorary title (~1 prestige per month)? Marry them to the daughter of an emperor (~400 prestige)?
For vassals who are already duke+, you can prompt them to create duke-tier titles easily enough: Give them all of the land in a duchy (eg. by transferring vassals) and ensure they have a LOT of gold (maybe 400-500). They should create the duchy title shortly thereafter, which grants 200 prestige. (Doing the same for kingdoms is more difficult, as creating kingdoms requires piety, and AI usually don't prioritise this.)
(NB: Everything except granting honorary titles will make your vassals more powerful, which will make your life more difficult in the long term.)