first mod i would suggest is the amber DRL's.
http://chevyavalanchefanclub.com/cafcna/index.php/topic,50150.0.html
then have fun...
Transmission - 4l60 to 4l80
http://www.ls1truck.com/forums/front-page-news/14163-how-4l60e-4l80e-trans-swap.html
Steel Slide / Bed Slide
http://www.steelslide.com/
http://www.truckspecialties.com/bed_slide_avalanche.htm
Under dash lights
http://chevyavalanchefanclub.com/cafcna/index.php/topic,128556.0.html
Truck bed lights
http://chevyavalanchefanclub.com/cafcna/index.php/topic,128779.0.html
Rear Door Lights
Kayak Rack
-remove factory crossbars
-Thule 450 Crossroads & 58" crossbars
LED Tail Lights
Transmission Cooler - done
Power Mirrors/Puddle Lamps
Magnuson supercharger, corsa exhaust, Volant CAI, long tube headers - ~ 100HP increase at rear wheels