Most travel agents in Tuk Tuk can sell through-tickets to your next destination, with the ferry crossing to Parapat included in the price. Ferries leave every hour from Ambarita and Tuk Tuk to Parapat. The first bus to Pangururan from Tomok (via the Tuk Tuk turn-off and Ambarita) leaves Tomok at 8am. From Pangururan buses leave every hour up to 2pm for Sidikalang, two hours away.
The trip from Toba to Berastagi involves two bus changes and takes four to six hours from Parapat. Firstly, you'll have to catch a bus to Pematangsiantar from Parapat (1hr; Rp1000), and from there a bus Kabanjahe (3hr; Rp2000) and then a bemo to Berastagi (15min; Rp500). An alternative is to catch the daily tourist bus (4 hr; Rp25,000), which takes in a couple of sights en route.
The first bus to Pangururam from Tomok (via the Tuk Tuk turn-off and Ambarita) leaves Tomok at 8am. From Pangururam buses leave every hour up to 2pm for Sidikalang, two hours away