Refractors were always my favorite, and they used to be fairly hard to get compared to these days. And of course, they are still around so their cool-factor has dropped a bit for me.
So now that I think about types of inserts that are still unique to the 90s and what my favorites would be, I can narrow it down to two types. And oddly enough, they are both Pacific inserts.
The first is the Cardsupials inserts, which is a regular sized card with a slot for where a small size card is kept. Here is a picture of one showing both cards, but this card is from 2000 and from online. Finding a picture with both the large and small card in it is pretty tough.
And the other would be the Christmas ornament cards. These were not just a card, but also have the string you can hang it up with. I do not have any of the baseball ones instead I have one for hockey. Here is a picture of one from online: