Easter eggs

Selectable marshal and plain texture drivers in driver options when holding Ctrl + ⇧ Shift keys
Training lesson editor accessible by pressing ⇧ Shift + T on main entry screen
Small kart track outside autocross area
Portable WC and buckets hidden in Blackwood pits building
The small crowd of few people, standing on an elevated stand placed next to the Skid Pad on Autocross (towards the "skybox" treeline, not the crowd standing on grass), is randomly and dynamically generated for each session (credit: maci3k (credit3k))

InSim packet infobox