Cruelty to Animals
At the rear bumper of your friend's car, attach a leash. When he drives around with the leash attached, people will definitely think that he's cruel and ties his pet to the bumper and has forgotten to take it off.
Pop a Balloon
This is definitely the most weird and funny prank idea, that you can try on your friend. Take up a large unfilled balloon and tie it over the whole of the exhaust pipe. After driving through a few meters, he'll hear a big POP! He surely will be surprised as to what has happened and you can laugh your heart out.
Jack the Car
Jack up your friend's car just barely above the ground, but, not that high that it gets noticed. Your friend will be awestruck, thinking in bewilderment, "Why the heck, the car isn't moving!!"
The mentioned ideas are some good pranks that you can play on your friends and watch the fun. They are absolutely harmless, yet, rib-tickling!