A backup drive ? dotfiles (.config) ? /tmp /var/log /boot ? map it into your home for something temporary - Downloads ? Slow audio files ? If you like to tinker for fun: btrfs / bcachefs could perhaps utilize it for something, like automatically offloading unused/slow files to the slow usb ? Raid ? ;) Create a partition and let zram swap compressed unused memory pages to the usb partition (I think it keeps pages compressed now ?). You can also run 'usb over ethernet', so another machine can use it directly - not sure why tho ;)

On a side note. I still use one of my first usb 1.? drives with 32mb in my old router. 32mb is still 12mb more than my first hard drive, and it fits well with the small openwrt packages. It even has a little slide button to select 'floppy drive' mode and a physical 'readonly' mode. I wonder what the smallest/oldest usb stick people still use, are ?