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You mentioned simple and cool... so here are the easy ones...
Waterfall emblem on coupes (not simple: you could paint your interior pieces as well, get custom seats, etc)
Exhaust enhancement plate is a must for every C5, I get pissed when I see one without. Rear lettering. Tail light grilles, if that's your thing (not simple: you could get a custom exhaust, spoiler, and rear facia screens)
Steering wheel emblems are a must as well, along with passenger airbag lettering (not simple: you could put in white gauges and aluminum bezels, also a custom steering wheel wrap)
Also, note calipers in sig pic (not simple: you could put in D&S rotors too)
That should be enough to **** your wife off for this year.
I have only done exterior mods so far, but the interior is next (Mod Red/ Black/ Gray)
So far I have done:
clear corners and rear side markers (with clear bulbs)
Z06 screens, air duct screens, side vent screens
new Cray Scorpion Wheels
CE convertible Stripes
painted calipers / Dimpled and Slotted rotors
tail light and license plate louvers (I like them, others don't- but it's might car, right! )
Corsa exhaust and C5 exhuast plate
I did the fog light screens, exhaust plate, and flush mount tail lights on my 01 mag red vert. They all look great with the Z06 titanium exhaust. Under $100 for the cosmetic stuff and $460 installed for the exhaust. Looks and sounds great for $500.
Exhaust enhancement plate is a must for every C5, I get pissed when I see one without.
I'm the complete opposite, I'll never put one on mine. I think they're just plain silly and the car has enough Corvette emblems plastered in enough places already.
Replace the air filter top with one from a 2002 ZO6, your car will breath (and sound) better, especially the early year C5s. About $50-60. No water problems either.