The other day I posted a staircase carpet magic item based on this picture:
Here is an alternative, suitable for any system.
When unrolled, the carpet creates a door to a pocket dimension. The dimension is a small, closed up rectangular unfinished basement, furnished with rickety chairs, a re-purposed table, and adequate lighting. Up to 12 people can enter uncomfortably. Each time it is used it re-stocks itself with 12 gamer-days worth of cheesy snack foods and caffeinated beverages. It can be used for up to 16 hours in a row, but then must be rolled up and recharged over the other eight. If rolled up with occupants in it, they are immediately (but safely) ejected.
Practically an unlimited amount of books, papers, dice, and miniature figures can be stored here. Any combat or exercise related gear, or actually valuable materials however, will be quickly buried until piles of character record sheets or messed up by cats and be effectively unusable.
The Portable Basement can be used daily by users up to the end of college. After college, it can be used once per week, schedule permitting. May require spousal permission.
Cost: Priceless. Weight: 5# but bulky rolled up.