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. 

Hint: Daytona: Easy win:
Make sure the race is set on ten percent difficulty. Then, cause a crash on lap four. If done correctly, there should be a caution. Go into the pits, and ignore that fact if no one else also enters. You will probably be last, but there will soon be another caution and the leaders will come in. Stay out and do not pit. After the stop, you should be in first or second place. You will run low on gas, but you will not run out. Block and you will be able to win.