The bubbling waters of Atami have been luring bathers as far back as the Nara period (710-94). Shogun Tokugawa Ieyasu (1543-1616) was so enamored of their healing properties that he tried to re-create the onsen back in Edo. The various onsen in Atami—there's not just one—have also drawn many of the rich and famous of the modern era. Izusan Onsen, one of Japan's most ancient, was discovered 13 centuries ago.
The Tokaido Shinkansen and Tokaido and Ito lines all go to JR Atami Station, and many buses go there as well.
To get from Tokyo to Atami, you can take the Kodama Tokaido Shinkansen, the limited express Odoriko, regular Tokaido trains, or the bus. The Shinkansen is the fastest way at 50 minutes. The highway bus is much cheaper but takes two and a half to three hours depending on traffic.
Shogun Tokugawa Ieyasu brought Atami's waters back to Edo to soak in
Atami has long been a favorite destination for couples
Atami's saltwater hot spring fame is reflected in its name, which means "hot sea"
The high-temperature water with its high salt content has excellent insulation properties, helping with neuralgia, and excessive sensitivity to cold. The water also tightens up the skin and tones the body.
Atami has a timeless reputation as a romantic getaway, and checking into one of the town's many onsen ryokan, traditional Japanese inns, is still one of the most romantic options around. The usual plan is one night, two meals, and luxurious bathing until you're wrinkled. While a stay isn't likely to be cheap, it will be memorable, and there are reasonable options available.
Onsen hotels are another option, and offer more in the way of entertainment, including karaoke, regular pools, games, and so on. Prices are cheaper than ryokan as well.
Although day trips and onsen don't usually fit together—since relaxation is the whole point—there are several places near Atami Station that offer baths both open air and indoors designed for bathing contemplation. There are private baths available to rent. There is even a footbath called Ieyasu no Yu in front of Atami Station named for the famous shogun.
The onsen manju, a kind of sweet dumpling, is probably the signature sweet people think about when they look for a gift to take back from Atami. The core ingredient is sweet bean paste. There are plenty of variations to choose from, including ones that incorporate bitter orange peel, barley flour, Japanese yams, brown sugar, tea, and even perilla leaves.
Visit Atami Castle for great views of the town and the Pacific Ocean, get your art fix at the MOA Museum of Art, and visit the Akao Herb and Rose Garden with its 12 themed gardens. The latter has a DIY workshop where you can make flower-themed items. The MOA museum has some magnificent works of art spread across a beautiful hill-top site overlooking the ocean.
Atami Castle, for all its glorious appearance, is really there for the view. It was built in 1959 as a tourist attraction. The ropeway is the easiest way to get there from the main onsen area at the base of the mountain and down by the seaside. There is a museum inside that tells stories about various castles around Japan, too. But most importantly, the view from the mountaintop, where the castle is built, is glorious.