Complete the Steal The Wheels mission to have racing cars appear.

Complete the "Make Elvez Sing" mission to make locked DEA and military vans appear. Once the game is completed some of them will become unlocked.

After several missions (after "the car dealer" at least), you will have a mission where a man asks if you want some tequila. If you accept, he will tell you where you can buy it. Once you reach the place, you will notice that is a trap. Kill the banditos and get out. A text message will order you to go to "the guy who set you up". When you reach him he will be fleeing, but the car is so old that you can follow him by foot. Follow him, but do not attack. He will eventually get out to attack you with a modern assault rifle. Kill him and take the weapon. Repeat as desired.

To get unlimited ammunition for the following weapons, you must collect the corresponding number of weapon icons scattered around the city.

WEAPON - NUMBER OF WEAPON ICONS
Unlimited grenades - 60
Unlimited hand gun ammunition - 70
Unlimited sawn-off shotgun ammunition - 80
Unlimited grenade launcher ammunition - 90
Unlimited rocket launcher ammunition  - 100

Unlock the following bonuses by performing the corresponding tasks:

BONUS - HOW TO UNLOCK
Mission select - Complete all missions.
Dual pistols upgrade - Collect ten weapon icons.
Dual rifles upgrade - Collect twenty gun icons.
Health upgrades - Collect ten health icons to upgrade your total health. This can be repeated.
Stamina upgrades - Collect ten stamina icons to upgrade your total stamina. This can be repeated.
Play as Ram - Complete The Good Son mission.
Play as Ram's brother - Complete The Father mission.
Bike in the Bull Ring - Complete the Kill The Bull mission.