I've turned an HP 2000 laptop with an Intel CORE i3 and a smashed screen into a hub for some old scrypt miners, as well as just a general purpose terminal in my 'noc'. If I had another one I would probably use it to learn more about, and set up, a honeypot on my home network.

Does it have a good GPU? Have it run password hashes for Aircrack-ng instead. Are you versed in Linux distros? If not, use it to play with unfamiliar OS (driver issues notwithstanding) without too much worry about breaking anything important.

#tldnr use it to learn something new