Get all 36 licenses in arcade mode, then press Circle at the stage selection screen.
Big head mode
Get 30 licenses in arcade mode with any character, then press Square + X at the character selection screen.
Board with Sound Effects
To get a board with silly sound effects, get all of the licenses for the L.A. level in another mode.
Bob Burnquist's board in L.A.
Get to the mansion. The board is on top of the car.
Bob Burnquist's board in New York City
Go to the roof with billboards. Grind from billboard to billboard. Land on a truck then grind on the wires to the building. Grind on more wires ghen jump down to get the board.
Carlos de Andrade's board in Ruins
Go to the platform where a big quarterpipe with the arch on the left is located. Lip Grind the quarterpipe over the arch and continue grinding. Stop at the wall and drop down to get the new board.
Chris Senn's board in New York
Go up the hallway (the way you get onto the billboards) to the next level. Then turn around and look down the hallway and the hidden board will be in the air. Wall Ride to get the board.
Chris Senn's board in San Francisco
From the start, go down the hill and take the first left turn. Go all the way down to the corner, where there is a group of extra bonus points and the fifth checkpoint. Do not grind to get them. Instead, go under the rail you grind on to find a house. The hidden board can be found on the left side of the house.
Collin McKay's board in Ruins
G to the area with zombies on a high platform. Jump off of the platform towards the waterfall.
Extra boards
If you have unlocked another mode, then you can get the extra boards. Play a level until you get all of the licenses for that level. For every level you do this, you will unlock another board.
Rick's board in Boat
Go to the first floor in the back of the boat on the grind rail.
Rick's board in Museum
In the third room, go up the slope and grind the dinosaur to get the board.