> accessing the internet is too broad. A C64 with a modem can access IRC and BBS > minimal modification/add-on This is a bit broad too. Every modern "add-on"

> accessing the internet is too broad. A C64 with a modem can access IRC and BBS > minimal modification/add-on This is a bit broad too. Every modern "add-on" has more power most of the old computer they are attached on. You could run a proxy on the add-on, that would shrink pictures, html and remove https. Or even simpler. The add-on could do all the computation and just use the old hardware to show the result.