Some fun things to do while driving:
Vary your vehicle's speed inversely with the speed limit.

Roll down your windows and blast talk radio. Attempt to headbang.

At stop lights, eye the person in the next car suspiciously. With a look of fear, lock your doors.

Two words: Chicken suit.

Write the words "Help me" on your back window in red paint. The more it looks like blood, the better.

Have conversations, looking periodically at the passenger seat, when driving alone.

Laugh a lot. A whole lot.

Stop at the green lights.

Go at the red ones.

Occasionally wave a stuffed animal/troll doll/Barbie out your window or sunroof. Feel free to make it dance.

Eat food that requires silverware.

Pass cars, then drive very slowly.

Sing without having the radio on.

Honk frequently without motivation.

Ask people for Grey Poupon.

Let pedestrians know who's boss.

Look behind you frequently, with a very paranoid look.

Restart your car at every stop light.

Hang numerous car-fresheners in the rear-view mirror. Talk to them, stroking them lovingly.

Lob burning things in the windows of smokers who throw their butts out the window.

While stopped at a light, pee out the window/sunroof onto other cars.

Paint your car with occult symbols.

Keep at least five cats in the car.

Root (cheer, not snuffle in the mud) for firetrucks.

Stop and collect roadkill.

Stop and pray to roadkill.