Glowing in good health
Onogawa is one of Japan's most radioactive hot springs, with radon concentrations of around 10 nanocuries per litre (200 Bq/m3). That's not quite as scary as it sounds though: worldwide average household background radiation is around 2.5 nCi/L, so a half-hour dip is no worse than two hours spent watching the TV. However, the water is also quite sulphurous, so expect a little eggy whiffyness.
Onogawa crops up regularly on lists of snowy hot springs, with a high probability of heaps of the stuff as early as December. It's reasonably accessible from Tokyo, boasts one of the better-preserved "olde Japan" villages in the north, and makes a decent base for low-key skiing in winter as well as firefly-spotting and hiking in summer.
The nearest train station is Yonezawa, two hours from Tokyo by Yamagata Shinkansen, while the nearest airport is in Sendai. Yamakō Bus  runs from Yonezawa station to Onogawa, but the service is very infrequent: six buses per day, running roughly every two hours from 8:05 AM to 6:35 PM. The trip takes 20 minutes and costs ¥600. If you miss the bus, it's around 3000 yen by taxi.
Not much, to be honest. In summer, fireflies (蛍 hotaru) can be seen at night near the river. There's also a little shrine devoted to the hot spring (湯の神社) and an even smaller temple of the "Tofu Jizo" (豆腐地蔵), but both are often snowed in and inaccessible in winter.
There's a hot spring (Amayu) somewhere in that pile of snow
The main thing to do in Onogawa is hot spring hopping. All inns sells a "Yumeguri Plan" (夢ぐりプラン) ticket that lets you sample any three baths in town for 1000 yen, either at hot spring inns or the two public hot springs, Amayu (尼湯) and Takiyu (滝湯). If it's outdoor hot springs you're after, Kajikaso (河鹿荘) and Kameya Mannenkaku (亀屋万年閣) will be your best bets, but check schedules beforehand as hours are limited (12 noon to 4 PM only at most inns).
In addition to full-body baths, there's a scattering of ashiyu (足湯) foot baths, which are handing for defrosting your feet after stomping about in the snow.
Komachi no Yu (小町の湯). Rustic little outdoor spring, separated into men's and women's sides, but with no facilities — even payment is on the honor system. Alas, even though it's next to the river, there are no river views from inside.¥200 (free if you're staying in town). edit
If you're here in winter, there's one more amusement option:
Onogawa Public Ski Course (米沢市営小野川スキー場), (500m from village center). Dec 25-Mar 15. With one ski lift and a single easy 500m downhill run, this isn't exactly Shiga Kogen, but at that price you can't really complain. Ski lodge/restaurant open only weekends.¥3210/day. edit
Most visitors will have breakfast and dinner at their inns, but there are a couple of simple eateries and izakayas in town.
Ryuge Shokudō (龍華食堂), . Tue-Sun 11 AM-8 PM. Cozy, smoky little den at the south edge of town, where locals come to slurp their noodles. Try the two local specialities, mamemoyashi ramen (with bean sprouts) and nameko-miso ramen (with slippery nameko mushrooms), and any of the tofu dishes freshly made by the tofu shop next door.From ¥600. edit
Kajikasō (河鹿荘), Onogawa-cho 2070 (Near bus terminus), ☎ 0238-32-2221, . The largest and swankiest digs in Onogawa, Kajikasō is a gorgeous traditional wooden inn with a jumble of stairways wrapped around a central pond and garden, given a thorough but sensitive restoration in 2007. One outdoor bath, one indoor bath and one reservable private bath. The gourmet meals here are a cut above the average, with plenty of local ingredients, but pay a little extra and treat yourself to some prime Yonezawa beef. Free transfer to/from Yonezawa stn once daily.From ¥8000/person. edit
Shirabu Onsen is only 10 km away by car, but getting there by bus will require backtracking to Yonezawa.
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