Lassiter is leading the investigation of a stolen ring and Shawn sneaks into the hotel to do some investigating on his own in episode 1x03 "Speak Now or Forever Hold Your Piece".
Source: Psych Forums
Juliet and Karen Vick join Marlowe and her friend for her Bachelorette party in episode 7x07 "Deez Nups".
From episode 2x11 "There's Something About Mira".
Liv and Clive investigate the murder of a magician during a magic convention in episode 2x07 "Abra Cadaver".