Re: What Mods do you regret?
Regret: waiting so long to get one (but I got a really good one when I did, so that is forgiven)
Undo: first aftermarket stereo I fitted (OEM one was impossible to read in daylight, wearing sunglasses, so I replaced it immediately) was fine, but a year later I upgraded to a modern NAVI unit with a backup camera, and Bluetooth, and hands free phone, and SD card compatibility (all of those are really handy, and bring the truck right up to date, electronic-wise)
First: LED dome lights (WOW), LED driving lights on the front bumper (also, WOW), if fitting 7" round like I've got in my avatar I recommend you use the PIAA mounting kit because it is awesome, simple and made of stainless steel (no rust).
also, supercharger (worth every penny, including the mpg hit you take forever afterwards, totally worth it)
Other potentials: If you go over speed humps very much, you'll notice the "milkshake roll" the rear suspension causes (whole cab rotates from side to side, due to the rear geometry, you hardly notice it under normal driving but speed humps make it happen very noticeably and I've got a lot of them on my daily commute). A pair of $80 rear spring air bags, set at minimum pressure eliminates it completely (hooray).
folding cargo tote for the rear area (awesome for carrying groceries, or any medium sized objects)
Undercoating and rust remediation!!! Deal with it sooner, rather than regretting it later. Lots of great threads on here with details. Definitely a "first thing" kind of thing to get done.