(Please Note: I know this isn't a huge amount of information, but I will try to upload videos and screenshots as soon as I can)
Floor 1: House without a door: Most people know about the house on floor 1 that can only be entered by the back door (good for trading), but there is another house that has no doors, but can be entered by glitching through one of the top windows. This house is located in the far right corner behind the central statue.
Floor 2 "Caves": secret "caves" are located throughout the Agnaroth Plains, presumably as a minor flaw in the design of the landscape. One cave can be found a short distance over a hill behind the Dire Wolf rock (there may be some wolves near the entrance) and multiple caves can be found underneath the shore near the Boar Forest. These areas are good for trading privately.
Floor 3 : There is an area that seems like an extended ground obviously a flaw in the floor 3 design. This area can be accessed by going to the wall behind the spawn panel then reach the top and walk left for a while then u will be able to reach it.It's a great trading area where no one will bother you.