Sandboxy approaches are great and I think your idea can work. I'm currently running similar (though instead of a snowed in valley, it's an archipelago). I think one important thing to do is to make sure that the players can acquire something solid that ties them to the region (in my campaign, one of the players has actually acquired a village). This gives them more of a vested interest in the area that you're detailing.

But keep in mind that no matter how detailed you make your locations, the party will just skip stuff or not look at it. Sometimes it can be disappointing, especially if you've come up with something particularly creative or interesting, but it happens. By the same token though, sometimes they will look at it and it can be incredibly satisfying for all parties when they do. =P

Also multiple paths are important, or lots of plothooks. Depending on how much is going on, it's very important to keep notes. I've had players bring up an incidental merchant I've thrown in before. Suddenly I've had to consider their history and background and stuff! But occasionally a party will need nudging in the direction of something. It's not railroading though as long as they have a choice in the matter, but it is needed sometimes.