Hollis Forsythe – Employee of the Year. Sort of.

After the end credits, Hollis Forsythe will tell you she received the “Employee of the Year” award. A quick glance will reveal that it is likely not real, but rather Dr. Loboto’s “award” from the beginning of the game. (Raz even comments, “That thing’s real?”) The text reads:
“this goes to the guy who is the best at doing the thing that is the thing of which you could be doing for a year.”
Additionally, the date reads “12-4” – that is, not the summer time in which the game takes place.
The Psychonauts 2/Adventure Time Crossover
... of sorts. If you re-enter Hollis’s brain after you complete the initial playthrough, you’ll find a large pen in the maternity ward. Grab it and throw it at the “Ward” sign, and you’ll get a weird scene animated and voiced by Adventure Time creator, Pendleton Ward (get it?!).
Accessing the Waterfall and Finding Lili’s Fungus
Use the Time Bubble on the logs in the Questionable Area to reach a hidden area behind the waterfall. Here you’ll also find one of Lili’s fungi.

Tim Schafer famously has a distaste for mushrooms. The line here is a callback to “The Secret of Monkey Island” where, when first entering the Monkey Head, Guybrush says, “I had a feeling that in hell there would be mushrooms.”
Dr Loboto’s Kid, Still at Camp
After you finish the main game, there is still a lot of exploration to be done in Psychonauts 2. One of the more interesting nuggets of information you can find is that Dr. Caligosto Loboto has a kid at Whispering Rock. Who can it be? Surname aside, Bobby Zilch looks to be a fan-favorite guess.