Something else that I've added that I like alot are the wind deflectors that go on the side windows. I got the kind that slide into the window channel, not the kind that stick on.
On a warm or hot day they really get the warm air out of the cab. Conversely on a cold day they help get the cooler air out too.
And, they look cool. Weathertech I think is the brand I got from MacNiel Auto online.
Another item could be clearance lights if he doesn't have any. They're $70 from a dealer for the kit that includes a template, or can be had cheaper in the aftermarket (just a bit over the budget you mentioned).
Aren't Geno's muffler eliminator kits right around your price range? Talk about a different sounding truck! I straight piped mine at a muffler shop for $40. Now it's nothing but smiles when I'm on the go pedal. Leave the resonator on though to keep the dreaded drone out of the truck.
Recently the wife bought me a dash mat for ~$35 from O'Reilly's. It will protect the dash from those "harmful UV rays" and cuts down on dust in the cab.
About a year back I bought a steering wheel cover made out of leather that stretches but doesn't neet to be sewn on. Cost about $8 from WalMart and I like it a lot too. Keeps from wearing out that leather that comes on the wheel, and it doesn't have any logos or whatever on it.
That's about all I've done to mine for under $50 a pop. Maybe a bug deflector for the front? Tint the windows? Good luck on whatever you decide.
Duane