Views vary on this, but the goal of a daily driven car are primarily safety, efficiency, and practicality. With this in mind, here are a few relatively inexpensive modifications you can make to your car to make it better at what it does.
- Better tires. The original author hits this pretty much right on. My only addition is to take local weather and road conditions into account. If you go for performance tires, you might need an extra set of rims with separate winter tires.
- Aftermarket air intake. Depending on your car, you can easily get an extra 1-2 mpg and a 3-5% increase in horsepower from an aftermarket air intake. While you don't need the horsepower in a daily driver, it's nice to know it's there. If you don't floor it all the time, the slightly improved fuel economy will pay for the cost of the intake in a few years.
- Fumoto valve. You really should change your own oil. It saves you money, and it gives you a chance to get under your car -- you might notice CV boots or other wear items going bad before they cause a more expensive failure. I never spent the money on a fumoto valve, but if you tried changing your oil and made a mess you might look into one.
- TPMS update. Most new cars have tire pressure management systems to let you know when your tire pressure is low. Unfortunately, most do not alert until your tire pressure is extremely low. Go to your dealer and ask if they can update your car's computer to throw a tire pressure alert when the pressure is 3-5PSI low. Keeping your tires properly inflated makes your car safer and makes it use less fuel.
- Rims. The original author mentioned rims, but with the focus of buying wider ones. Instead, consider getting lightweight alloy rims in your stock size. If you run different tires in the summer vs. winter, use the light rims in the summer to see an improvement in fuel economy. Wheels are heavy and unsprung weight, so in theory you should see a significant improvement.