CGI-based images or pages. I don't believe GeoCities let users upload their own CGI scripts, but I believe they had a set of pre-defined CGI tools you could call. You could also set up things like guestbooks using an external service. It wasn't unusual to see "Guestbook provided by $FOO" at the bottom of guestbook pages. Kind of like Disqus, but before JavaScript.
Yes, I remember these external services but in those cases you had to link to a page completely off the site.
Geocities had some feature where you could actually embed guestbook entries into your page.
Tripod let people write their own (limited) perl scripts for their site... when I found that out the first thing i did was try to get something from Matt's Script Archive to work, but it didn't. Then I wrote my own guestbook and what had to be the lamest forum ever.
Truthfully I don't remember for sure, but I used external services just as @Lexarius mentioned. I didn't write it myself at the time.