Paper Insider has a free paper model of the time traveling steam train from Back To The Future III. I really can’t tell what the scale is, but from the side panels, it may be ok for 25mm.
Delta 7 Studios has a collection of free paper models of the Seven Wonders of the Ancient World: the Great Pyramid of Giza, Hanging Gardens of Babylon, Temple of Artemis, Statue of Zeus at Olympia, Mausoleum of Maussollos, Colossus of Rhodes and the Lighthouse of Alexandria.
Your science fiction troopers need a utility hovercraft to get around in. Here’s a nice paper model of just such a craft.
For Infinity and other science fiction games, here’s a paper model of a Chickenhawk dropship. It comes in a couple of different versions.
This one is really cute. It’s a paper model of a Buck Rogers looking girl on a retro style rocket.