One of the most popular things to do on vacation in Japan is head to an onsen or "hot springs". They're dotted all over the country, and people—both young and old—enjoy visiting onsen towns, where they eat delicious food and bathe.
When taking a bath in Japan, you wash first and then enter the water to help keep it clean. Unless you are in a private bath, you usually bathe with total strangers. The baths are usually divided between men and women, but little kids can go in with either parent.
Parents go with kids. Friends go together. Grandparents go with their grandkids. I could never imagine taking a bath with, say, my grandparents in the U.S., but it doesn't seem odd here in the least.
The hot springs are relaxing—just as a long, hot bath should be. But, depending on the spring, the water has different prosperities. Some springs bring good luck or long life.
I quite like going to hot springs—they're extremely relaxing. In the past decade or so, I've noticed that they become more and more accommodating to foreigners who do not speak or read Japanese, which makes them a primo destination if you are visiting Japan.
And the food? More often than not, delicious.