I use my spare as kind of a media controller sort of speak thanks to kdeconnect and casting locally. In the near future, I want to get into emulation so that might be what the spare phone gets used for instead with the help of one of those cheap steam deck docks for USB C. As for more creative ideas, I heard some people use them as a dedicated GPS, media player or even a dashcam for their cars (not sure how the battery would hold up in the heat and whatnot). You can use it to remote view your pc or even play games from that same pc or even one you don't own through cloud gaming. You can repurpose it to be a 2fa device only, so you can keep your TOTPs and other 2fa methods separated from your main assuming the device is up to date security wise. If the camera and whatnot are decent maybe keep it as a spare camera for all your video/picture needs (online meetings, video recording, document scanner, etc...) a dock might be useful for that. If your spare has that desktop like experience when you connect it via HDMI there's nothing stopping you from using it as a really lightweight computer to do basic web stuff. These are really vague answers since I don't know what phone you have but I hope it helps get the ball rolling