Recipe books scattered throughout the game that teach you everything you can do with turnips. Collecting them all reveals an ultra secret spell. Practicing Jan's formulas also makes you evil.
Jan is the only companion I legit can't stand out of the two BG games. Only ever brought him along in one playthrough just so I knew what he had to offer. Managed to keep him around up to the end of chapter 6, then kicked him out to do Watcher's Keep.