The Instant Fortress has already been mentioned. RAW is unclear as to what happens to the stuff inside.

Building a tiny room inside a Portable Hole has also been mentioned (although the link to the loft uses some 3.0 material from The Stronghold Builder's Guide).

If you have The Stronghold Builder's Guide in play (3.0, though, so....), well, there's an entire section on making your stronghold self-propelled. Do note it gets expensive quickly.

There's an entire section on Transport as well. Stormwrack has a full section on boats, but they don't fly by default. Eberron gives you Airships.

If you have your own Demiplane (from Genesis), you can get there and back regularly. An Amulet of the Planes or a Cubic Gate gets you on and off the plane itself, and Boots of Teleportation (or the Helmet) can get you back to where you were.