you wont gain much of anything mod wise on the 3.8L engine, but you can do a few cheaper things to help it out.
Gears are the best bang for the buck, they wont add any hp or tq but they do help acceleration a TON, much more than any other simple bolt on by FAR, go 3.73 if manual 4.10s for auto and expect a half second quicker 0-60 time (or about like bolting up 50hp lol). Hurts gas mileage on the interstate a bit but helps in the city~
Exhaust is everyones opinion, the 3.8L is an even fire motor and simply does not sound good above 2500 rpms with a loud exhaust, so you can stay mellow tone OEM gt mufflers or make it "loud" that is not always good, its an opinion there so go with what you like.
Other than that you can get a cheap intake ~