I have done a bunch of mods on my 2014 gt. Most noticeable is the pedal commander to improve throttle response, then goodspeed track springs, Bilstein struts. Sway bar ends are 14mm in these cars, they wear out after 20k miles if you drive aggressively. I swapped mine for Moog 22mm sway bar links. Huge improvement in handling and eliminated any shake in the wheel.
I have lots of other mods, aes sealed air intake, borla stainless cat back, but really the pedal commander made the most difference in driving experience.
What is "relatively cheap"? There are no real meaningful gains in power that are cheap. The best money spent on engine performance is add an intake and axleback system then do a tune. Other than that you are just throwing your money away. That Peddle commander thing is just a box that changes the pedal position signal to the ECU and tricks you into thinking you have more power. The $300 or whatever it costs would be better spent on real tangible power gains. My 2.0L car is pushing 190 hp at the crank, up from 155, with a tune from OVT, with a virtually flat torque curve from about 2500 to 6500 rpm.
Other performance must haves are a rear sway bar upgrade, proper lowering springs (H&R, Eibach, Racing Beat) and dampers to match (Koni Special Actives seem to be a very good match for this car).
Depending on your mileage, an oil change to synthetic, air filter replacement, or changing out worn spark plugs could refresh your car and restore your driving experience. Even a nice wash & wax could make you love your car again. However, modifying a car tastefully is best made using quality parts.
I would start by asking yourself what kind of driver you are and what you value most out of your car. Do you favor MPG over the thrill of driving? Or perhaps you prefer to cruise around town with quality music playing? Or you are looking to make your car more flashy so it stands out?
Decide what's most important to you right now and what you want to change first: utility, appearance or performance. Then decide what you can afford and just get started. This will cost a bit in the long run but it is a hobby and hobbies are generally expensive. In the end you will have a personalized car that you will be very proud of.
But whatever you do, don't rice it out. It will destroy your car's value.