How to Unlock Sandbox Mode
Start the Breakroom scene (very beginning of the game). During the force grab tutorial, throw all items into the left side Archive receptacle.
Continue through the rest of the level and continue through the next museum level until you are about to enter the physics playground that requires jumping. There is a secret archive door in the corridor leading to the playground. Inside is a Boneworks box. Break it open and take the module inside.
Take the module with you to the area with a vending machine. To the right of the vending machine is a receptacle meant for guns and items. Throw the module into it and finish the level.
The module is now available in the main menu and all you have to do is plug it in.
How to Quickly Unlock Arena Mode
You’ll need to get the sandbox key first:
There is a crate in your Main menu that you break open and get the Sandbox Key. Well, it’s actually a universal key. On the main menu where you would plug the key. there are two plugholes.
One that says A Fantasy World, the other says Sandbox. The Sandbox key is universal and you can plug it into either hole and then play that mode.
You have to plug it in, launch the mode. and the next time you go to the main menu put the key into the other plughole and launch that.
After you plug them in one time, the option stays unlocked so you don’t need to keep switching.