Things you can put in a spare drive bay

https://www.amazon.com/dp/B01IE37ZRU

Four SSD drives

Quad density drives: one bay can hold four.


https://www.amazon.com/dp/B01LWSP4K8
Card Reader+

Display with USB 3.1, Multi Card Reader, Fan Speed, and Voltage/Amperage LCD


http://www.frequencycast.co.uk/plusdeck.html

Cassette drive

Voice, music or data.


https://www.aliexpress.com/item/New-Computer-5-25-Inch-Drive-Bay-Storage-Drawer-Box-Tray-for-DVD-CD-ROM-PC/2000883591.html

Storage

Turn your computer into a tiny dresser.


http://www.crazypc.com/products/8177.html

Cup holder with lighter

Coffe cup holder and cigarette lighter.


https://www.amazon.com/dp/B00KJGYLNM

Fan Controller

Monitors and controls multiple fans.


https://www.amazon.com/dp/B008BC1RHO
Watercooling reservoir

With watercooled systems you can hold cooling fluid here. Perhaps if your computer doesn't run exceedingly hot you could use it for a small aquarium. A very small aquarium.


https://www.amazon.com/dp/B00126U0VA

Hot swappable drive bay

If you need to swap drives regularly this works.


http://www.thinkgeek.com/stuff/41/ezbake.shtml

EZ bake oven.


http://www.crazypc.com/other/misc/toast.htm

Toaster.


http://www.thermaltakeusa.com/Product.aspx?C=1257&ID=1481

Touch screen display