Well I've already got my existing 64GB iPad 2 which is downgraded to iOS 6 and it's essentially a giant old game console, full of old iOS games and emulators.
The iPad 3 I got today works well as a second display (via TwomonUSB) so it will take that role from my iPad 2. Having a touchscreen portable monitor is just severely underrated. It'll mostly be at work with Teams on it, and occasionally with my study laptop as a portable second display.
The other two iPads are both Air 1st gens. One of them will be used just as a device for me to watch YouTube and browse Lemmy on in bed while my main phone charges.
The other one I'm yet to do anything with but I do like the ideas others have commented. Might be used as a self hosting dashboard showing uptime, usage stats, internet reliability, something like that. May also prompt me to actually set up a self hosted NVR