Lets take a vote. For me
 

Im 15 and about to get my first vehilce (99 blazer ls) and i beleive i can get a free mod on it from the guy at the dealership. What do you think i should get?

A) Pipes/exhaust
B) Cd player
C) Grille guard
D) Tire rack
E) Mud tires
F) 2" lift

Its a 1999 chevy blazer ls 4x4 and its sittin on 31" tires and the windows are tinted but, has stock cd player and street tires. And if i get a grille guard i can work my way to a winch and help out the stuck peeps ;D

my vote is for the guard....but i have to ask....is it a full guard or like a bull bar? just curious

Its already on 31's? So you're getting a ZR2? I hate you -__- lol

I would go with tires because they are the most expensive of the options.

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Well not necessarily. If he gets a suspension lift and you add in labor, it will be about what mud tires would cost. If it's a suspension lift, I'd go that route personally. Just my .02.

Edit: I'm kind of confused. One of the options is a tire rack, yet if it was a zr2, it would already have this. OP, can you specify whether it has the zr2 package or not?

Yeah i just saw that as well. Hmm but i thought all 4x4 2DR blazers had the external tire carrier

Depending on where most of your driving will be I would stay away from true mud tires. Usually 2 door blazers have the tire on the rear swing arm and 4 door carry them underneath behind the rear axle. Either way since you're in high school looks and sounds seem to matter. I would start with either the sound system or exhaust. Good luck.

A dealership is giving you aftermarket parts for buying a car?

The tire rack was taken off and its just a 99 blazer ls 4x4 doesnt have the zr2 logo.

Mudders are horrible on gas! Look cool, got that big truck, off road road sound to them & unless you can afford the repair bill from offroading every weekend they aren't worth it. AT's are much better. Grille guard looks good but for functionality ehhhh good place to mount lights & look cool.
A lift isn't needed because you already have 31's on an LS but if you're going to add bigger tires in the future then sure, get it.
Now a sound system should pop with a highschool crowd as would the exhaust but what is the exhaust? get a single in/dual out muffler thats a straight through muffler. I know, I have one & trust me.... it sounds great! I have truckers come up to me at work: Hey man I saw you down the street the other day leaving a stop light - holy chit that thing sounds good! Is it a V8?
A single in/dual out muffler with pipes to the rear should cost around or over $600 FYI

I'd go for the new exhaust. Good for sound and performance.