So this is a true Boner of a game and right up my alley. I'm crazy about secrets in games and even more crazy when it's presented as a major part of the game.
I took screenshots of all (most?) secrets that I found, and figured I'd share them here and ask what little secrets everyone else has found!
Starting area
Right on top of the structure you start the game at. You end up with a witch hat for the rest of the game.
On top of the castle (kinda difficult to get to), there's a Haunted PlayStation logo and a skeleton dude who brings you to a pretty big bonus area if you hold onto him.
...But if you let go before you get warped to that bonus area, you can end up on top of the middle part of the castle! I can't seem to get there exactly as there's an invisible border, but it seems one of those bald heads lives inside the crown. Additionally, some other picture on the inner wall??
This is the closest I could get to the inner wall pic and I'm laughing my ass off.
(Edit: I zoomed in a little and the small text says "I am in control of my own destiny")
Skeleton man's bonus area
Anyway, the bonus area also has this little cube below the platform you start on. I'm sure there's, like, global level events tied to it but it'd be pretty great if it actually did something if you approached it. Unfortunately it doesn't.
The "Game not saved!" bit is pretty good too.
Castle
Going into the castle, if you turn back, it seems you cannot go back out of the castle but you will instead encounter this little peebling hanging around in the darkness.
Further into the castle, at the four-way split where you have to get the blue and green keys, there's a hole above the monkey king's fountain. It turns out you can manouver your way up there and encounter another one of these bald heads.
Ice area
Well, you can tear the snowman's head off and there's another little peeble.
You can find your way on top of the snow hut and there'll be another Haunted PlayStation logo.
Going into the snow cave, there's this obvious secret where you can get the red key and the monkey head mentioning a hole in the green area...
If you fall down onto the other low platform in that large cave area, there's a kitty cat.
The Green Area
Well, there's the little room with a lever behind the room with the giant spikes... It appears that it opens up the entrance a little later on where you can insert the red key to open the hole.
The hole leads you to a crazy Monkey Ball-esque bit. Couldn't really find secrets in this area but the "Ball Out!" is amusing.
Then the game resets so make you you had your game saved by quitting to the main menu before this point.
Back to the green area...
Near the end, if you turn around and jump down from the right side, it seems that a very long pole is attached to the giant spike from the previous room. And there's something red at the very end of that long pole... I really cannot tell what it is. A red hand, maybe?
The Castle (part 2)
This is the point where I learned that if you Boner at this character, he'll halt whatever he's saying and look offended for a few seconds.
Also, this is really interesting! Since you can skip killing the monkey king by going into the secret area above the castle, ENT_01 can end up not saying anything about Peeb killing the monkey! For some reason, the crazy rainbow patten inside the knight was missing in this instance.
The Cool Checkerboard Place
I only found one secret, but it brings you to a whole new area. Just walk right through the fence on the right side of the big open area.
The Test Level
If you want to enter this area more than once, make sure to SAVE first, since entering this area a second time will get you stuck in there forever.
Enter the portal shown in the previous screenshot and you get into a silly "test level".
You can walk right out of the holes in this level, fling yourself around, and find another one of those white cubes hanging around below the starting point.
That's about all of the secrets I found in the levels... But finally, this also happened to the main menu at some point. It seems to happen randomly. Everything gets dark and ominous and pressing the Boner key will play a very distorted version of the sound clip.
Anyway... Thoroughly enjoyed playing this and really looking forward to the full version!! :) What have you guys encountered?