Gero Onsen's symbol is the frog, not surprising, since "Kero" or "Gero" is the Japanese equivalent of "Ribbit." You'll see froggies everywhere, and there are plenty of onsen as well, most of them strung out along either sides of the wide Hida river. For a large, industrial onsen town it's remarkably charming, perhaps because so much of the tourism is still onsen-based.
Gero is conveniently located -- buses run to and from, and it has its own train station. It's a good spot for those interested in visiting Shirakawa, a popular old-style village, and in season, you can watch cormorant-fishing on the river. Though you won't find yourself alone in the mountains, you'll still find a trip to Gero makes a great escape from Tokyo...and is fun year round.