Put Linux on them and give them away to people who need them?

Turn them into a little server that you can host self hostable services on

IMHO fix whatever you can, donate it all locally (HackerSpace, RepairCafe, Linux non-profit, etc) as there are quite a few people dedicated to refurbishing computers for schools, people who need a computer to find work, etc.

Then for the tinkering aspect, keep one, that’s enough.

Honestly even 1 isn’t really required. Pretty much everything listed here can be done more efficiently without an actual physical computer :

• your current computer can be a server, just turn off the screen or even accept (which I’d argue is a fair assumption) that at night it will be off. If you want external access put WireGuard or another VPN on it.
• Want to test distributions or anything else? QEMU or containers, no need for actual hardware

Some ideas:

• webserver (e.g. for a little personal website, maybe even host some fediverse things)
• irc
• weather monitor
• distro tester
• local LLM ~(they’re getting more and more efficient)~

If you’ve several of similar performance, you could:

• host lan parties, for classic games. Maybe some Quake, OpenTTD, Luanti

Cheap Linux desktops for a charity?

I have some hardware from like 2008 running my entire home’s infrastructure. Jellyfin, Kavita, home automation, etc.

First of all: get rid of the broken ones. You’re not doing anything with the running systems, so there’s no need to hang on to the ones that don’t run.

Next, make a list of the things you want to do and start doing them.

If you’re worried about power consumption, don’t be. If you’re still worried about power consumption, get an inline watt meter (a kill-a-watt), take some measurements, do the math and feel at ease. If you don’t feel at ease, look up wake on lan. You can have powered down computers turn back on when they get a packet so you don’t need to worry about power consumption.

When you feel like you’ve done enough stuff, get rid of the computers you’re not using.

Solar panels and cryptocurency mining

Explore weird OSs! I got an old Celeron D workstation just for playing around with weird old operating systems.

Its got a 32 bit bios but 64 bit celeron, so the grub stuff has been fund 😅

A suggestion: if you can’t find anything else for them, keep them around as parts machines.

There should still be useful components in them. For instance, a lot of the Wi-Fi modems may still be perfectly good for other things as long as they’re mini-PCIE (I don’t know if they use those in desktops). They may not be the absolute newest standard, but should still do the trick; it certainly came in handy when my sister’s laptop’s Wi-Fi modem decided to be a brat - I just swapped in an Intel modem from a laptop from 2016.

I might not fully trust the SSDs or the HDDs, but they can still have their uses. There’s one SSD from an old desktop that I currently have hooked up to my Wii U.

How old are these machines, from oldest CPU model to newest CPU model?

A lot, depending on your interests and the hardware itself. I’m running a NAS (TrueNAS) on an old machine that also runs a bittorrent client and immich as TrueNAS “apps.” I’m running an *arr stack and jellyfin on another old machine. I’ve got another old machine running an i2p router, hyphanet node, and a few other services. In the past, I’ve used old machines as routers (pfsense), openhab/home assistant machines, game servers, ZoneMinder server, etc.

I recently turned every old junker and some nicer ones into a Harvester cluster. The really old ones I use as cold storage devices that I actually shut off when I don’t need them.

Install Win7 to make retro gaming machine?

Turn them into a Kebernetes or a Proxmox cluster.