LAN file sharing — download and upload, both toggleable, from any device on your network.
Turns this computer into a small LAN file server. Other devices on the same Wi-Fi/network (phones, laptops) can browse and download files from a shared folder, and optionally upload files back into it — both toggleable independently, live, without restarting. Pick the folder to share, scan the QR code (or type the shown address) on the other device, and you're connected. No installation, no admin rights, no system changes. Note: there is no login and no encryption, so only share folders you're comfortable with anyone on the same network accessing, and turn a mode off when you're done.