The ODD74 forums also recently resurrected discussion of a post on JDJarvis's blog that I remember, but didn't comment on: basing an entire campaign on two pages of notes. Presumably, this just means the political and geographical layout and any magic or monster differences; individual dungeon maps would be extra, although the names and available knowledge of such dungeons would be part of the two-page rundown.
To a certain extent, that's how I ran the Nine and Thirty Kingdoms campaign. Of course, the two pages were mostly in my head, and I probably drew more area maps than I needed to. Which all makes me want to go back and condense the entire thing, maybe using the techniques for improv towns to keep the starting town descriptions short.
The way I see it, the trick to making a fairly detailed sandbox (sketchbox) campaign starting with only two pages is to stick to two or three broad statements at each level of detail and only expand things in play. You don't need the entire world; you just need enough nearby, far, and very far place names, eras, and cultures to drop into conversations with NPCs. And for villains and patrons in the area, you don't need a whole lot to start out with, or very much detail; just a name, level, short descriptive phrase, goal and obstacle (or peeve,) one or two of these per major locale. You can look that list over during play to decide what events will be set in motion ... not to create a story, but to create a background for what the PCs are doing, which might or might not intersect with or even make use of the "non-player events".
Wherever the PCs are right now, you're creating a current map, fleshing out details as needed using these broad statements. Draw a map for one significant dungeon near the PC home base, have a couple names and sketchy details of other, smaller dungeons. You can use a couple d6s with pips to create the basic layout of these small dungeons and stock them when needed, or maybe use a simple rule like "pull one random entrance geomorph and one other geomorph from a collection, add a few extra tunnels and rooms, and stock." If the player announce they are going somewhere new and they end the session traveling there or after investigating a randomly-generated entrance, you can complete it in a more satisfactory, less random, way before the next session.