The trans cooler is the best "must have" and is relatively cheap (especially compared to a new tranny). We're of course assuming you have an auto.
A TBU would be cool, but I wouldn't call it "easy".
The ISR mod is pretty easy and gives you some extra peppiness. You can also do the elbow removal mod.
If your set on interior, you can do the LED upgrade. Start with maps, dome, and cargo. Then work your way into the dash, head unit, and shifter. You can also custom install lighting in the footwells, cargo hatch, etc. You might also consider the head unit upgrade to a 99+ so you get the better cupholders. If you have an aftermarket radio and the rear antenna (look at the passenger window in the cargo area), then you can do the antenna mod. You get a Y connector and attach the two antenna wires into it and attach that to the radio. Some say you get better reception, I haven't gotten there yet. You also figure out a way to hold your rear headrests if you fold your seats a lot. Lots of threads about that. Do you fish or use long poles for another sport? You can do the bungee cord thing inside the roof.