The first thing, how is maintenance? Oil, Plugs, Trans Service, any suspension parts worn out?
CAI is not that big of a gain. Improving the airflow from the filter to the throttle bodies would net the same. To have a true CAI would require alot of effort such as moving the charcoal canister and ecu out of the way so the filter can drop down into the fender more. Personally I suggest opening the opposite side of the airbox open (facing towards fender) adding a K&N filter and replacing the accordion style tube with decent piping from the box to the throttle bodies.
There is no reprogramming for our cars. And with it being electronically controlled swapping injectors will only improve in a small window of things. AKA WOT (wide open throttle) where the duty cycle is near 100% on injector flow rate. However you can get more air into the engine by boring out the intake and swapping those twin 38mm for 52mm 3.3 throttle bodies. But that does requires alot of work.
Exhaust is all a manner of personal choice. These engine rather you have the 3.3 or 3.5 are not big enough for a nice tone like a v8, however At low rpms and higher rpms mine sounds freaking awesome. But alot of annoying rasp in the 2.5k-3.8k range. I have the 3.3 however and OHV engines always sound different than OHC engines.
Here is decent vid to hear my exhaust..
One of the biggest things you can do is get a Aux. Trans cooler. This helps out on so many levels for keeping your car happy always a huge upgrade. Why I have this huge thing on mine..
The biggest things you can do are Factory Upgrades, such as OTIS (Overhead Travel Info System), leather seats, and the biggest one AutoStick. A/s will bring whole new level of enjoyment to your car far as drivability. Also, there is a 300M PHP brake upgrade which is considerable amount of work and parts but worth it if you are a spirited driver.
There is a bunch of ways to go far as upgrades and personalizing your ride. All just depends on which way you want to go. Handling? Power? Comfort?