Ideas to upgrade car?

So I bought one of those arduino car kits (2WD) and mounted it. Currently the car has 2 motors connected to a motor driver, bluetooth module so I can use an app to control it, an ultrasonic sensor and external battery with an interruptor. I also have the rotary disks and encoder sensors. ← (anything cool I can do with these?)

What are some interesting modules or features I can add to the car?
I have many different modules like lcd displays and sensors.

Thanks.

Add some red and blue LEDs and some kind of noise maker and turn it into a police car.

The rotary encoders are important for measuring distance travelled and the relative speed of the two motors (they won't be exactly the same). To make the car go in a straight line, you need to use encoders to continuously monitor the motor speeds and adjust them to be equal.

To me, the most interesting project to build would be a line-follower robot. Some ir emitters & sensors to sense a white or black painted line or tape on the floor. Once you have the car following the track reliably, including sharp turns and intersections, you can lay out a maze, and have the car find its way through the maze without previously knowing the maze layout. Have you seen the "micromouse contest"? Search for it on YouTube. Amazing!

The rotary encoders are important for measuring distance travelled and the relative speed of the two motors (they won't be exactly the same). To make the car go in a straight line, you need to use encoders to continuously monitor the motor speeds and adjust them to be equal.

Wait, is this required even when I'm manually controlling the robot? (movement commands set by code, lets say that when I press the forward button, it goes at 200 in analog) do the motors not go exactly the speed you set them to go, so you have to implement a controller?

I'll also look into the line tracking.

Wait, is this required even when I'm manually controlling the robot? (movement commands set by code, lets say that when I press the forward button, it goes at 200 in analog) do the motors not go exactly the speed you set them to go

Imagine the lubrication on one wheel is slightly poorer than on the other, or the axle is binding slightly more, or the wiring to one motor is a bit thinner, or the soldering isn't quite up to scratch. . .

When you are manually controlling the car, you steer it to go in a straight line. You can see when it's not going straight and you correct it without thinking about it. But when it is steering itself, it needs feedback from somewhere to know if it is going straight, and that is what the encoders provide.