In the Guild of Archeologists, there's a magical alarm system on the door
that leads to vault 51. If you try to enter without disabling the
alarm, you'll experience an interesting transformation.
Paintings
In Pseudopolis Yard, check out the paintings on the upper level.
They may be familiar if you've played Discworld I & 2.
Mirror
Writing
In the dressing room, check out the reflection of the names in your
notebook. Some get funny responses.