I set up a second mini PC with Linux Mint XFCE edition which I found for only $70 used (it's a bit old but still has an SSD) and which uses only 15W of electricity. The XFCE edition is ideal for older hardware.

One piece of software that makes this setup extra useful is barrier, which is a free virtual (software) KVM switch, i.e. lets you use the same keyboard and mouse on 2 computers that are both turn on, and switch between them without unplugging things:

https://github.com/debauchee/barrier

Setting up barrier took a bit more effort that their page indicates - you also have to ensure that one computer always has the same IP address by configuring your router (in my case, go to http://192.168.2.1, login, Advanced setup -> DHCP reservation). And I had to disable SSL in the barrier options to get it to work.

Since I play PC games, my other computer pretty much has to have Windows and be powerful and consume a lot of electricity, and I think a lot of people are in my situation. The advantages of having a second Linux mini-PC beside it is to save electricity when I'm not gaming and be able to use Linux for all other tasks besides gaming, without needing to reboot whenever I want to switch between a game and another application. A virtual machine is another thing people do but I think my setup is better because VMs are slow, it saves electricity, it prevents me from having to deal with the annoyances of using Windows all the time, and it means I have a spare computer for when one dies.