I turn all my old laptops into servers. Most often, I use them to experiment with various self-hosted services.
Most laptops these days can be put into clamshell mode, especially if they have an HDMI out. Just get a headless dummy plug -- they run about $6.
Plug one of those in, close the display, and you have a small, book-sized server with a built-in continuous power supply backup battery.
Of course eventually, the power consumption of all these devices can add up. Older devices are often less efficient. Sometimes, just retiring and getting a device "recycled" (to the extent possible) is best.