Bullet proof PCJ-600
In the mission "Biker Heat", when you chase down Wayne, kill him WHILE HE'S ON THE BIKE (he cannot fall off the bike or it will not be bullet-proof anymore), steal it, and then save it for a bullet-proof PCJ.

Bulletproof Stretch
You must be on Staunton Island and on the mission "Search and Rescue" which is given to you by Sal over the pay phone. Go pick up the limo from Sal's garage, then drive the limo to YOUR garage. Do NOT go to the yellow marker. Go straight from the Sal's garage to yours. After the limo is in your garage, fail the mission (die or blow up the limo while it is in your garage) and then the Bulletproof Stretch is yours. This works with other mission vehicles as well.

Rampage without getting wanted
Start a new game. Instead of taking Vincenzo to the Safe House, leave him in the car and don't get in. Now you can roam free and do whatever you want without getting any stars at all!

$50 Good citizen bonus
Just like in Vice City, you can receive $50 as reward for helping cops apprehend criminals on the street. Occasionally, you'll see cops chasing criminals on the street. Most of the time, these criminals happen to be gang members. No matter who they are, just do a melee attack on them and you should receive $50 as good citizen bonus.