Thunder Plates
Select the Thunder Challenge or Lightning Challenge at the game mode screen. Complete the various challenges to get new Thunder Plates. NOTE: The Plates are awarded randomly from game to game -- winning a certain track with a particular driver will not unlock the same Plate between games.
Night tracks and alternate tracks
Race at the desired track in quick race mode under the veteran difficulty setting. Win the race to unlock a night or alternate version of it.
Paint schemes
To get all the paint schemes for all the drivers, race with the desired driver. Select any track in quick race mode under the veteran difficulty setting. Win the race to unlock one paint scheme. To get more, go to another race track or the same one at a different time (for example, Bristol day or night). This will not work if its the same time of day, such as the two Richmond races or the two Loudon races.