Hello :)
 

Hello from Bulgaria. I drive an old Volkswagen and I am not going to change it for any price because it is my very first car :)

Welcome to the forum.

There are lots of mods you can do which are reversible or don't alter the appearance of the car.

Or you could just focus on improving your driving technique!

Just adjusting the nut (behind the steering wheel) will gain you the most! That and jacking your tire's PSI's just a little!

Don't be a chicken!!!

Welcome to the forums!
Larrybuck and MetroMPG said it all, but I would definitely second pumping up your tires. I gained more than 2 MPG between my first and second tanks of fuel after buying my new car, just by pumping the tires from 30 PSI to 42 PSI. Just stay at or below the max sidewall and you should be good.
Start a fuel log if you haven't already. Happy Ecomodding!