Weathertech mats! I leave mine in all year, the stock mats will look nice as fuck when I go to sell or trade the car.
Window tint. Go to a good shop, and ask them to sand the dots on the 1/4 windows before they tint it, or it will get that dumb looking line around the dots. I always recommend getting the best film possible, as it is something you look at and through every time you drive. I got the sides done in 55% and the windshield in 80%. The car cools off so much faster in the summer sun than before. Plus, it's way easier to convince a passenger that nobody else will see.
Not exactly a mod or enhancement, but I use fuelly to track my mileage. Link is my sig, while it seems frivolous to keep track like I do, it helps determine when a problem occurs. My VW dropped like 4-5 mpg for a few tanks before the engine went. I cared so little about it when it happened, I started to throw money in savings towards a down on a new car instead of fix it.
It's also a nice thing to hand to the new owner if you sell the car private party. I know I would be stoked as balls to see somebody that tracked fuel economy on every tank, and had all records of oil changes and service history from a dealer. Car might have higher mileage, but it's been taken care of since day one.