The first view of Crater Lake will literally take your breath away. The water is such an intense blue that seeing it is a surreal experience. The reason for that azure color is because of the depth of the water: this is the deepest lake in the United States. It's also one of the cleanest lakes in the world. The lake formed 7,700 years ago when a volcano exploded and the top crumbled. The cavity filled with rain, and to this day the water level is maintained solely through precipitation and melting snow.
There are several viewing points around the rim of the caldera, as well as hiking trails. Swimming is allowed in the lake, but the water level is only accessible via a steep trail. Fishing is also allowed. Although there are no indigenous species, several fish were introduced in the late 19th century until 1941, when the fish stocking ended.
Because of its elevation, activities are very seasonal. The campgrounds are open around June through September, but it's always best to check in advance if you're near the beginning or end of that time period. Crater Lake Lodge and The Cabins at Mazama Village are also open seasonally. They're both located in Rim Village, a complex of accommodations, gift shops, and dining.
Admission to Crater Lake National Park is $25 in the summer and $15 in the winter for passenger cars, or free with an American the Beautiful Annual Interagency Pass.
Because of its elevation, Crater Lake is frequently invisible because of clouds. The National Park Service has a webcam so you can see what the conditions are before you visit.