The first real mods on my Miata were suspension and wheels. I put on a set of Ceika custom coilovers built for the NA, which let me lower the car and adjust ride height and stiffness instead of running the tired stock suspension.
The wheels were a gift from Andy: a set of silver and black XXRs that came off one of his cars and fit perfectly. With wider tires, the car got more grip, and together with the coilovers it sat lower and handled better.
Andy and I installed everything together.