There are portals to other ages of history here.
For example...Dungeon of the Ages. It seems to just be an area of ruins. Then they go through a doorway into a greco roman temple complex complete with an arena. Then there is another door, beyond which is the inside of a pharaoh's pyramid. Farther on there is a tunnel entrance to a cave age area where primitive cave men are being invaded by couatls, celestials, elves, and other advanced races from the sky. If they dare go further they will end up in the age of Dinosaurs.
They can turn back at any time, and return later, because these warps are here permanently.
Futureland. Nearby is a cinder block warehouse with a cloaked door in the back that leads to an old west general store in a town rife with outlaws and gunslingers. In the jail there is a strange metal security door that leads into a modern office building. In the cellar is a secure door with a keypad that if picked leads to a domed, automated, seemingly abandoned city in the Spacefinder game. Outside the dome is an inhospitable wasteland with mutant plants and animals adapted to the poisonous atmosphere.
If settlement: The portals are in doors, possibly locked or secret. Both magic and technology are at +6.
If no settlement: There are ruins and archways. A portal might be in the back of a cave.