Hiya.
Your first guess was what I was going to suggest. The KotBL is an excellent module (probably my all-time fave), and I've run "sandbox style" campaigns with it that lasted more than a year. ALL the play for that campaign was in/around the KotBL "wilderness map". Good times, good times...
If you want more "story stuff", you could grab the Hackmaster 4e equivalent, "Little Keep on the Borderlands"; basically the same thing, but turned up to 11. A LOT more dungeon, far more "NPC stuff" going on, more detail about the wilderness areas, etc. Might be very hard to find though...
Maybe look at the first and maybe second adventure modules in Paizo's adventure path "Kingmaker" (
http://paizo.com/pathfinder/adventurePath/kingmaker ). I ran the first one and the PC's kinda/sorta started the second, but they decided their characters didn't want to take on the burden of ruler ship, so they said "It was nice, and thanks for all the fish!" and then just up and went north for no particular reason...

But the map and area info would make for good baseline-sandbox stuff. They put out a Map Folio that has the maps for the AP...and that particular map folio has gotten high-praise.
If you are looking for a "dungeon/underground based" sandbox, you would be hard pressed to find anything better than the old Basic D&D "The Lost City". Excellent dungeon adventure with plenty of interesting NPC's, topping it off with an underground city in a vast cavern for you to develop further. Oh, and a bunch of "further adventures" ideas. All this can probably keep a campaign going for years I'd wager. Never done it myself, but maybe it's time....hmmm....
^_^
Paul L. Ming