I've been to Sasaya cafe a couple of times now, for lunch at the weekend, and it is a really lovely little cafe. Located in Shinsui park it's very easy to find. Take the Asakusa Line metro to Honjo-azumabashi and exit at A2. When you get onto the street, just keep walking straight until you come to the entrance to Shinsui Park on your right (about a 7 minute walk, you'll go past a 7-11). Head along the path in the park, following the water. After about a 10 minute walk, you'll see the cafe on your left (it's inside a large park building, but you can see the signage in the window)
There is such a nice atmosphere in this place, it's very family-friendly. I've missed breakfast (served until 11am) but I am eager to try the tofu cheese toast, so I will be going back. I have had a couple of the lunch sets, which include a small cup of soup, salad, and choice of drink. I had the veg sandwich - delicious bread, hummus, lots more salad - and also the fried tempeh, which came with rice and was amazing. Very filling and a good price, 993 yen. They also have a small shop that sells coffee and some natural products. Unfortunately, they close at 6pm so it isn't a place for dinner. But it's close to the Skytree,so if you're up that way, give it a try.
Cons: Closes at 6pm