I have done a few small things to the box like added my Sirius radio, new tires, Denso Iridium plugs and the Viper 791xv from my last car but the best investment by far is the "Night Light System" (Part number: 545T) http://www.directed.com/security/access/oaccess.asp (bottom of the page).
Its made by DEI, the makers of car alarms and starters http://www.directed.com/

Basically it turns your head, tail, running & dash lights on for you when it gets dark using a small photocell mounted on the dash.
It also has a daytime running light option that turns on just the headlights when its light out. It's a feature that my Chrysler LHS had that I really missed.
Tho it's made by DEI it works stand-alone, it doesn’t need a DEI starter or alarm.

I had my Viper 791xv installed at the same time as the 545T. I bought the 545T from the installer for $79 so I paid top dollar for it but it can be had off eBay much cheaper.
The 545T was $20 to install it while the 791xv was $125 to install.

From a safety standpoint the DRL feature is worth the price.