Lake Toba is home to the unique culture of central Sumatra's Batak people. A recent article in the in-flight magazine of Garuda Airlines (yes, that's the one blacklisted by the FAA) offers a look into unique place. Samosir Island is a surprisingly large island in the middle of Lake Toba. The article offers a glowing summary of the place:

Samosir is inseparable from Lake Toba. The island, with an area of 627 square kilometers, lies in the middle of a stunning tourist location, Lake Toba, surrounded by lovely green valleys. Exploring the island brings the visitor into contact with a unique, unspoiled culture, with traditional houses, tombs, and the activities of the local people, all painted in the blue of Lake Toba's waters and the green of the surrounding valleys.

Think Toba sounds a little too good? In general, Sumatra lacks the infrastructure of Indonesia's other main island, Java, so getting around can be an adventure (an ulcer inducing adventure). Still, if a relatively unspoiled and natural palce is what you are looking for, Toba and Samosir Island are worth a look.