Dungeons and Dragons

There was a campaign a long time back where I gave the party something called the "Infinite Tapestry." It was kind of a low-grade portable hole, a 10'x10' tapestry that depicted an image of an empty stone masonry chamber with bare walls. If the tapestry was hanging vertically and you touched it while speaking a command word, you would appear inside the tapestry where there was another similar tapestry on the fourth wall of the chamber showing the outside world. Both tapestries had to be hanging flat and vertical for the connection to work, otherwise the other tapestry turned black and temporarily inert.

Basically, it was a portable clubhouse. They furnished it, went on a minor quest to get ahold of a magical hot tub to put in there, used it to store all the unwieldy things that parties liked to lug around. Since the connection was relatively "fragile" and could be disrupted easily, trapping whoever was in there until the tapestry could be hung from suitable supports again, they were always careful about using it as a literal camp - they slept outside of it whenever possible and took care not to all be inside at the same time. Which was a bit frustrating because there was a secret to the tapestry that I really wanted to force them to discover.

It wasn't a portable hole, the chamber wasn't an extradimensional space. The tapestry was actually a two-way portal to the Feywild, specifically to a chamber in the basement of an ancient abandoned castle that was the ancestral home of the secret Big Bad of the campaign. But they never examined the masonry inside to discover that the "back wall" was slightly different from the rest (it was a bricked-up corridor leading to the rest of the basement) and circumstances never arose where they'd be able to determine that they were on a different plane of existence in there rather than an extradimensional space.

Eventually, though, an opportunity arose. Through sheer happenstance a minor enemy of theirs discovered the secret of their tapestry, including the command word. He went into the tapestry to search through the party's stuff, but the party spotted him entering it so they took down the tapestry to trap him in there. They spent a day prepping to fight the guy, and then when they were all good and ready they put the tapestry back up... and saw that during the day the guy had spent trapped in there he'd figured out the thing I had wanted the party to eventually figure out, breaching the back wall and escaping into the castle in the Feywild.

So now the party had to dungeon-crawl the ancient castle chasing the trail of this guy, discovering a bunch of stuff about the real Big Bad of the campaign in the process, and once that whole multi-session extraveganza was finished they had graduated from having a simple "clubhouse" to having a whole castle in their pocket. It became their major base of operations and many future adventure seeds focused around it.

It was a lot of fun, but if you're a DM that likes a well-organized storyline it might be troublesome. I had no idea when exactly they were going to discover that whole new branch of the campaign they were carrying around with them.