One of the greatest aspects of module B1 "In Search of the Unknown" is how readily the Caverns of Quasqueton lend themselves to expansion.
Access to the lookout tower, a tesseract among the maze of doors, and random actions for the statue on the lower level were previously discussed in B1 expansion ideas on Dragonsfoot, back in 2013. Here are some additional ideas:
This volume appears unremarkable at first glance, seeming to be a notebook with many handwritten entries of undecipherable runes and markings. It is actually a diary kept by Zelligar, and it details one of his adventures from the distant past, written in his own hand. The writing is not discernible unless a read languages spell is used.
The larger jar is of clear glass and seemingly contains a black cat's body floating in a clear, colorless liquid. If the large cork lid is unstopped, the liquid will instantaneously evaporate, the cat will suddenly spring to life, jump out of the jar, meow loudly, and run for the door. If the door is open, the cat will dash through and disappear. If the door is not open, the cat will be seen to pass through the door and disappear. In neither case will the feline be seen again.
Another (more gruesome) explanation is described in Jon's D&D Vlog #19. Jon pursues his idea to its logical conclusion, worth checking out.
Accessible only through a secret door in the Room of Pools, the chamber of Marevak, advisor to Zelligar and Rogahn, contains various documents:
The desk in the room is mostly empty, except for several attached sheets with various notes written in elfin. The first sheet is headed with the title, "Suggestions for the Further Development of Quasqueton," and the notes relate to certain details of construction for the stronghold (although there is no information of a sort to assist the adventurers, and no maps). The document (discernible only by those who know the elfin language or by a read languages spell) is signed at the bottom of each page by Marevak.
Just outside the Cavern of the Statue, in the corridor which leads eastward, is a large covered pit at the intersection of three corridors.