Not just mouse, keyboard, monitor, speakers.... you want nice ones, people underestimate how much cheap peripherals matter and will often use whatever they got for cheap but these are the parts your senses interact with and therefore have a big impact on how much you enjoy using your computer.
Magnetic USB cables for keyboard and mouse (only if the mouse is wireless), tie or attach the mouse cord so it can't flop all over and it makes it almost like a dock when you need to charge, just get close and it does the rest. These lower the stress on cables and ports, some look pretty nice, and they make swapping out easy and fast. I also keep a spare on the desk for charging because one cord (instead of 3) can charge Type-C, Lightening and micro B devices.
A desk hub, my current one has usb ports and a power switch/light but previous ones had power, reset, light, drive light, usb, sd card reader, esata and at one point a resource monitor. It allows you to put the system out of the way and still access these functions/features. Companies make them but mine were made on a 3d printer.
A good desk chair.. Get rid of that crappy gaming chair or whatever you're using. Check Craiglist, Offer Up, Facebook, etc... The best deals are often on Leap v2 but sometimes you can find Herman Miller for cheap. You may scoff at a $200 or $300 used chair but that chair will last years and your back will thank you. Warning though, if you use crap chairs your back will hurt until you adjust to a proper seating position.
If you have lots of power dips (lights dip), get an uninterruptible power supply (UPS), it will lower the burden on your power supply and make your system last longer. If you overclock it will also make your overclock more stable.
A monitor shelf is also pretty nice.
