I guess it depends on what you want to use your truck for.
I ride quads, so I put in a spray in bed liner (Line-X) for the back, and weather tech mats so I don't get mud all over the inside
I also like the storage space in the bed, so I went with an access rollup tonneau cover to keep things dry.
I wanted to keep people out of the bed, so I put a pop lock on the tailgate.
flip down DVD player for the kids
Intune tuner and K&N intake for more HP and better mpg.
HID kit for better lighting.
LED reverse lights and bed lights were a nice addition for cheap.
my antenna would drag my garage door, so I got a shorter one.
everything served a purpose for me, but if you are looking for ideas lookswise:
I want to get some wider tires, but will probably keep my stock AT wheels.
painting the GMC signs is cheap and easy way to clean up the looks.
some people like to de-badge everything, which just takes time and gives a cleaner look.
Grilles, LED taillights (someday if they ever come out with some for the 2014's), and aftermarket headlights (again waiting for) are ways to give a personal look.
level kits, or lifts add a lot to the looks (some even lower them)