Dramatically beautiful, Lake Toba is the sort of place that people come intending to stay 3 days and end up staying 3 weeks. It is a perfect place to stop and unwind, take a break from travelling and relax. Not that there is a shortage of things to do. There is trekking, or you can cross the island on foot with an overnight stay in one of the two Guest Houses on the top in the village of Dolok Swim in the lake, rent a canoe or catamaran, or try your hand at fishing.
Rent a bicycle to explore the area - there are footpaths which take you to hidden Batak villages where very few tourists go. Or rent a motor bike and take a trip around the island. If you do this, it is better to go around clockwise, starting toward Tomok That way you will do the most difficult part of the trip when you are fresh. Sarnosir is the size as Singapore Island, but its roads are not so good. Many people underestimate how long the journey will take, (about 7 hours) and if you go the other way you could end up coming down a dirt road in the dark. Besides, the views from the top are huge and it would be a pity to miss them because of failing light. An easier option is to drive the 45 km on a good road to Pangururan via Ambarita and have a long soak in the hot springs.
Money changers can be found at Tuk Tuk and in Parapat, but there is only one bank, the BNI in Parapat, which has an ATM that accepts Master Card/Visa. There is a big traditional market in Parapat on Saturdays, a smaller version on Tuesdays and Thursdays, and a nine hole golf course, green fees Rp 20.000 Mon-Thu, Rp 30.000 Fri-Sun. Or of course, you could just relax, sit on your private balcony overlooking the lake with an ice cold beer and a good book, and think how lucky you are.