Seconded.
Here's a vt100 terminal emulator that supposedly runs on a 128k Mac. There are many recipes out there to enable a tty on a serial port in OS X, and a vt100 terminal is feature-ful enough to run "alpine"/"mutt" for email and "links"/"lynx" for web. (And with "screen" you can even run both at once.) All this software is easily installed on the OS X host with Fink or MacPorts. If you need to transfer files to the Mac you can even run xmodem (assuming your terminal program supports it) from the UNIX command line.
The end result will basically be the same as if you'd logged into a BBS account. If you really want the full experience there are plenty of text-menuing shells out there for UNIX that you could set up to offer direct single-key access to the various programs you want to use after logging in.