Short throw shifter is a relatively cheap mod and will make the car a helluva lot more fun to drive. I'd recommend MGW. This is assuming you're stick.
Exhaust is a must if you don't already have it. Look up different setupd on YouTube to see what you like. You could get an offload h or x pipe and then a cat back system that you like.
You could also get a tune to free up some HP and make her faster. If your auto the shift times will be tuned too.
Both of these mods will make the car more fun, cooler, louder, more enjoyable and are definitely worthwhile
- 1998 Mustang GT Convertible: 5-Speed, 2001 PI 2V swap, o/r h-pipe, Magnaflow catback, MGW shifter,
3.73 gears, Auburn LSD, TA diff cover, MM FLSFCs, Ford Strut Tower Brace, J&M SS Brake Lines, and a lot of visual/restoration work.