You certainly can. Both ecosystems allow you to connect things to the onboard pins to provide extra functionality. For the Pi they are called Hats (sit on top obviously) and the Arduino has Shields (sounds tough!). So many options exist for both, from audio amplifiers, screens, more type of monitoring sensor than you can dream of, a camera for the Pi that turns it into a smart home security system, Really, the actual board itself is just a jumping-off point these days.
Here are some of the best accessories you can get for your Pi right now