Hello, my dear young addicts.
I'm here to help you out a little bit with your completion of the game the fair way.

I just suppose, that probably you didn't notice (because I didn't until like 5 hrs of gameplay) some small headcrabs randomly spawning somewhere randomly on the screen. What I've figured out is that clicking on this headcrab gives you either a ruby + some money or just a bajillion of money.
The same will happen if you click on something similar to a fish. Finally, in the game there is something like a rabbit, which can appear below the title which tells you your current zone. This guy needs you to click many times on it to get a random skill activated for free (the best one it can give you is a Dark Ritual, because it has a cooldown of 8 hrs). This rabbit is the same as a bee.

I hope that my advice will help you, fellas.
Good luck! :)

P.S. Headcrab on the Beach Princess is not clickable, it was just added as a part of design in Steam version.

Headcrab











Rabbit










Fish











Bee

• Edit 1 (17.05.15): later in the game you'll find some more secrets, but headcrab is the first one.
  
• Edit 2 (23.05.15): added screenshots of a "rabbit", "bee" and "fish" secrets.
  
• Edit 3 (23.05.15): changed word "hefty" into "handy" in the guide's name.
  
• Edit 4 (23.05.15): all "edits" and screenshots were put into new columns.
  
• Edit 5 (14.06.15): added "P.S." line.