Although maybe not what you'd think of as an interior mod, having a quality full window tinting done offered me the benefits of: Interior stays cooler (keeps my dark leather seats from getting hot), less glare from bright headlights in side and rear view mirrors, security in that it is almost impossible to see what's in the rear seats and back area, glare and heat from the sun coming in the side windows when driving is greatly reduced.
Factory tinting does not offer any UV protection, but quality film tinting does, supposedly 100%. My plastic and leather seats will get less sun damage. Also, having a custom sun shade for the windshield really helps protect the dash, and keeps the car cool when parked. I spend the winters in Arizona, so this is a must.
Best mod I ever did to any car. I had the darkest tint installed that is allowed in my home state. Doesn't really affect vision looking out, even at night.