Lake Toba is a very beautiful and astonishing lake, it's the most pretty place on earth I think. And if you like to spend your honeymoon on Lake Toba you could stay in a small peninsula which attached to the island. You may even experience stay with the native people of Tuk Tuk village. The air is vey cool and you may try to dive in the lake. Don't forget to bring some souvenirs back home like beautiful woodcarving in low prices.
Another destination to visit is Lawang Hill or Bukit Lawang. Bukit Lawang is a spontanious tourist area that located in Langkat, North Sumatera. About 80 kilometers from Medan and aproximately 68 kilometers NorthWest Binjai. Bukit Lawang is a part of Gunung Leuser national park. There is a rehabilitation center and conservations of Orangutan. You may join the activity to feed the Orangutan. I think it's cool activities during your honeymoon.