I think the cup holder is the most commonly replaced part.

replace the cup holder with a storage bin about $30

If you plan on keeping the car for awhile, you should stock up on motor oil when there is a sale. Mobil1 0w40 is about $7 a quart and it takes between 8-8.5 quarts depending on who you ask.

List of things I've replaced:
cup holder
Steering Stablizer
Rear differential (warranty item)
Razor sharp Door Pulls