In the "Destruct and Serve" mission, destroy the big silver sports car and a message saying "You have found an easter egg" will appear. Now when you complete the mission, you will get an additional 1,000 coins.
Although trophies can be gained in story mode it is far easier to get them by playing multiplayer and customising the game to suit the trophy you are trying to get, e.g. use the Crates Everywhere Wormpot modifier when collecting crates.
Win 17 deathmatch battles in a row to unlock the challenges maps.
Unlock all of the character sets and you will unlock this scheme in the shop. It costs 2,500 coins.
Each of these costs 150 coins and will be unlocked after completing a certain part in story mode.
Buying certain weapon schemes in the shop will unlock a weapon that kinda resembles it. Each of these schemes costs 150 coins and the unlocked weapons cost 150 coins each.