There are a lot of fine comments in this thread. Excellent advice! My thoughts:
The lack of "autoscaling" is indeed a benefit of sandbox play. What if all the players had a bad day at work? OK, take your 8th level PCs to "Kobold Hill", which you have ignored to this point. I'm not going to say that the kobolds all became Barbarian/Monks overnight. They're kobolds, and kobolds are supposed to be physically weak (though crafty, of course). The fight will probably be a cake-walk. Why not, if that's what the PCs want to do? The silver coins won't amount to much, but everybody got to show what a bad boy they are... which they wouldn't get to do if all the monsters autoscaled to the PCs. When you're 8th level (or whatever) you should get to kick a little gratuitous bootay now and again, if that's your pleasure.
The tougher areas ought to be further away from civilization, or otherwise reasonably marked off from incursion by lower level PCs. The most voracious monsters don't live next door to the starting town or village... otherwise that would have "come to a head" long ago. Rather, the worst critters live out in the "here there be dragons" districts... the Outlands, the Forbidden Wastes, the Plague Lands, etc. Even if something is discovered near the town, if it's a more challenging site it should be called "Tomb of the Lich King" or somesuch, or there should at least be available rumors which allude to its toughness.
Sandbox play requires the players to do their homework. They should seek to gain information about locales they intend to visit, or which their expedition will pass near. If they don't, shame on them. Likewise, if a 3rd level party insists on trekking all the way out to the Crag of the Gargantuan Lich-Dragon, the DM should happily serve up the entree they've just ordered.
Sandbox is all about player choice. But with increased freedom comes increased responsibility, as they say. So the old line "Well, it wouldn't be here if we couldn't beat it" doesn't hold. The players are responsible for knowing when to retreat, to regroup, to hire mercenaries, to abandon the expedition, etc. They're also responsible for setting goals and being proactive. Perhaps it appears that the Lord Chancellor is trying to take over the kingdom. They could help him, oppose him, or ignore the situation. All three will have different consequences. Maybe they ignore the situation, the Chancellor tries a coup and the coup fails (I'd make it a percentile roll, when the time came). So nothing really changes. Or maybe there is a major change. But the players can do as they will. Maybe they helped the Chancellor, then found out he was a jerk, then skipped town as his plot went down the tubes. So they'll be avoiding the Capital City for a while. It's all up to them.
To me, this style of play is very attractive. But of course it puts a burden on the DM to actually cook up a decent realm and have interesting things therein. Though if you buy a published setting (like Mr. Gygax' Castle Yggsburgh) much of the work is done for you.