but what other cool things can I do with this?
That depends on your imagination or need, and ability to write a sketch. It helps if you know at least the basics of C++, or some other object oriented programming language - but the instructions at
https://www.arduino.cc/ starts you off from scratch. It can be a lot of fun, and an interesting way to automate tasks, make a robot part, game controller or any of ... i dunno, a million different things... There are people who have built systems to manage their garden, built fancy light shows right into clothing... the sky's the limit. A friend of my youngest learned how to make a working airlock in a day. It's not just about the arduino itself, it's also about the motors, sensors, switches... and putting them together to solve a problem.
Also just to add to what
@Bruttle spoke of: You can even marry several Arduino devices with Raspberry PI devices. Leverage the processing power of a computer, and the process control capabilities of the Arduino... and you can even get intelligent devices. I wouldn't use it to drive an autonomous vehicle, but the technology isn't all that different.