The best Mario Kart 8 pre-order bonus doesn't even require a pre-order. Many Target stores are offering a free, official MK8 Wii Wheel with purchase. I got mine as soon as my nearby store opened this morning. It has Mario on it and everything! (No Mercedes logo, though.)
Supplies may be limited, as the Target I shop at had only three Wii Wheels available for the promotion. Naturally, they had just six copies of the game. Go figure.