When you ask that question, you need to define what is a "mod"
Someone even consider throwing an all-weather floormat on the floor is "mod".
By my definition, if it can be easily taken out and does not affect the fundamentals of the vehicle, it is not a "mod".
Seat cover, floormats, power inverter, cargo nets, cargo mat, etc. (you get my points) are NOT mods.
That said, I have done some to my Mazda3.
Accessories:
Rear bumper stainless plate, door sill plates, rear window tint 30%, splash guards (f/r), wheel locks, and most lately rear deck spoiler.
Planned:
K&N filter, power-folding mirrors (if I can get them from Japan), lower side chrome, etc.