Owakudani is an active volcanic valley. I have heard that it was known as "Jigokudani" (the Valley of Hell) by locals some time ago. Once you step foot on its wonderous soil, it will surely will take your breath away. I am not just talking about the magnificent view of Mt. Fuji, but also the rising volcanic gases that originate underneath the valley itself. When you look around you, there is an infinite amount of steam pouring up through vents scattered around the mountain, and a very distinct odor that will remind you that you are indeed traversing a very live volcanic valley.
There are a few hiking trails around the Owakudani station, but there is one trail that you can't miss: The upward trek to the egg boiling site. It's about 15 minutes uphill, and well worth the effort. At the top, you will get the opportunity to feast on freshly boiled black eggs! Black eggs, you say!? Don't worry, they are regular chicken eggs. The albumen and other parts of the egg still look the same as a traditional boiled egg, but it's the manner in which they are boiled that makes them a little bit special (80 degrees Celsius for 60 minutes in natural spring water. Then steamed at 100 degrees for 15 minutes in steel baskets.) The water that they are boiled in contains sulfur and iron. The black eggs, or commonly called "Kuro-Tamago" by the locals are perfectly safe, although they may smell like sulfur. In fact they are believed to add a few years to your life span -- five to seven years depending on who you talk to. I don't really recommend anyone finishing up to five eggs by themselves though!
When I visited Owakudani, five eggs were selling for 500 yen. Some people will hike up the mountain to eat the eggs at around 1050 meters above sea level. But you can also buy them from one of the stores in Owakudani Station.
There are plenty of other omiyage (souveniers) to choose from, if you are not too keen about bringing home the eggs. There are manjus (bean jam buns) that have been designed to look like the black eggs, as well as tasty cookies!