And to avoid technicalities, make it big enough so you can see and use it.
The most obvious solution would be make a small PC: more powerful and bigger fans means less noise. I've been considering something like this (https://www.youtube.com/watch?v=Jr5MjhgPz_c)... but then how am I supposed to use it? Yes, I can ssh into it, but what if it fails to start? Just last month my Thinkpad server failed to restart properly. This was a trivial fix but it being a laptop whose lid I can just open and use immediately made it an extremely easy fix, which would not be true for a PC.
Thing is, I know that dumb terminals exist, ie, a screen, keyboard, and trackpad that takes the form-factor of a laptop but has no actual internals, it's just a convenient interface when plugged into a server. I've seen them. I've tried searching for them but there doesn't seem to be an agreed-upon search category, and the ones I manage to find are more expensive than the PC itself and are usually designed as a server-rack drawer.
Genuinely, what do people do here? Do they just have their server setup somewhere like a desktop? Or are people keeping spare monitors, keyboards, and mice around that they then need to unpack, plug in, and use awkwardly before putting it all away again?
or really any cheap IP based KVM is what you’re looking for
I'd say it's needed about once a year at most though. Servers don't just fail to start, normally.
Around me, most days I can stop at goodwill and get a monitor and keyboard for $30 or less.
The issue isn't cost in this case, it's the storage and effort of having to lug it out and put it back afterwards. Even if someone gave me an old screen and keyboard for free, I'm still not going to build that server PC. I've been looking into PiKVM as advised by another comment and they're pretty pricey at ~£200 but that's genuinely orders of magnitude more preferable. In another conversation on another platform, I was told about nexdock, which is more for docking phones but can be used as a dumb terminal, which is pretty enticing... though their website is pretty dubious, eg: the shop doesn't even tell me what version of the nexdock I'd be buying.
Then connect it once and leave it. :p
Since you're mentioning opening laptop's lid I assume you mean literally failing to start, as after power cycling. For that, wouldn't simple hitting the power button be enough? It certainly doesn't require keyboard. If you plan to place it somewhere not easily accessible, there is Wake on LAN, which most modern PC motherboards are going to support.
If some maintenance task cannot be done with ssh/tmux, you can always use remote desktop software, in local network even RDP will do. And if something went wrong enough for you to not be able to connect to the server remotely then there is indeed no way around bringing and connecting a spare keyboard and monitor, but events like that should be quite rare normally.
They're stored together in a small box with all needed cables, so they're easy to take with me to whichever computer is having issues. In practice I only use them a few times per year.
In the performance window of "old Thinkpad", why not go fanless? Those lovely little Intel N150 mini-pc boxes are mostly fanless and completely silent - I have on my desk running Jellyfin/web server/etc, and it's inaudible under load.
> but what if it fails to start?
In ~15 years of running headless linux boxes, I've never had one crippled to the point it wouldn't boot as far as ssh.