In what way can I still use an old, still functioning laptop since I bought a new one? [closed]

You could consider donating it to a local school or Goodwill. You could consider using it as a media player / server with something like KODI if it has HDMI, but that could be more trouble than it's worth if you already have an AppleTV, Chromecast or Roku. There's really a nearly endless number of possibilities, but it depends on the value of your time and also the power efficiency of using an old laptop. For example, you could use it to act as a network attached storage (NAS), but that's way overpowered and uses far more energy than a dedicated NAS or using a low-powered computer like a Raspberry Pi. Again - donating it to a charitable cause or underserved community could be best.