What should I add to my '90s website?

Nice start, this is very nostalgic! If you ever had an old Geocities, Tripod, or even MySpace back in the day, check out the Way Back Machine and look for that old URL for some inspiration.

I recommend the following suggestions to build upon this better:

• Good if you can make this an HTTP site rather than HTTPS, although I'm not sure if that's possible on Neocities.
• Fix the font choice - back then in CSS you would list a series of fonts that would render - Arial, Helvetica, Tahoma, Verdana, Times New Roman. It's too legible, so switch it to a color pink that is a bit harder to discern from the background. You need black rectangle backgrounds for some of the text.
• The framerate on the GIFs is too high. More pixels and less frames - preferably no more than 15 FPS if possible. I used Macromedia Fireworks back in the day and some other Macromedia animation program, good if you could have terrible color profiles on some of the GIFs).
• Affiliates sidebar linking some sites. It may be good to link to other retro 90s-inspired sites. They're out there, but I can't think of any off the top of my head.
• Include some fake banner ads and sidebar ads - not pop up ads as those were the bane of existence. Maybe old banner ads for Rack Nine web hosting, or something similar (and of course they shouldn't link anywhere).
• "Jump to Top" anchor hyperlinks.
• You need a low quality midi that plays in the background automatically - no user choice.
• Add a Favicon.
• Plenty of links on the page that don't go anywhere. And more under construction signs.
• Adjust the resolution of the background tile - it's too big and too high quality. Get it to something like 128x128 or 250x250.
• Add a Shoutbox. Remember those? Although I'm not sure how to keep it secure.
• A link to "Bookmark this site".
• You need a fan art page that only has 4 fan arts, says "under construction", and possibly steal other people's fan art and credits them so it looks like there is actually content on the page while hiding the fact that you have no time to work on this web site but you're hoping a million visitors will one day come here.
• I already see marquees which is great. Good if you can another marquee that has text moving in a wave/ripple pattern as it slowly (and keyword: very, very slowly) moves across the screen. I don't remember if you could have alternating color text as part of it, my old HTML, PHP, and CSS knowledge escapes me from 20 years ago.
• A link to your PHPBB or Invision Power forum that, when clicked, only takes you to a blank white screen with the typical MySQL error. The hyperlink should have a badly inserted text next to it that says "This is currently down, we will fix it with a new Forum software after Finals Week!", the more ASCII art the better. I recommend something like the below.

General Error
SQL ERROR [ mysql4 ]

Table './that90ssite/phpbb_sessions' is marked as crashed and should be repaired [145]

An sql error occurred while fetching this page. Please contact an administrator if this problem persists.