From Penang in Thailand it was a long ferry ride to Belawan, Northern Sumarta in Indonesian. These back packing trips had all sorts of transport modes, one of my favourites was this cool motorcycle with side car ride, complete with army like helmet from the ferry port to Medan before catching a bus to Lake Toba.
From Pradat it was a ferry out to an island in the middle of Lake Toba called Samosir.
From Lake toba it was more time on the local buses and headed to Bukit Lawang.