Yotsuya is an old neighborhood and is not touristy but it does have some places to visit like St. Ignatius Church, Sotobori Moat Park, Kioi Hall, Shinjuku Historical Museum & the State Guest House.
Shinmichi-dori – is a street renowned for its izakayas and other interesting food eateries that are very popular with locals. Take Exit 3 from the Namboku line, turn right & cross the road, turn left, pass Family Mart & Haifukiya Drug Store, Shinmichi Dori is on the right.
Tôsôen is run by two women from Laos with Chinese descent. They serve dishes from Thailand, Vietnam, Laos, etc.
We ordered Chicken Satay, Gapao, Padthai & Beef Curry. The herbs, seasonings & condiments used, enhanced the flavors & believe me, it is the closest you can get to authentic South East Asian dishes in Tokyo plus the food portions are reasonable. The Satay is sinfully delicious !
Do not expect a fancy restaurant with great service. It is a simple, cozy, warm, down to earth place.
They don’t have a website but here are some details in Japanese ~ http://www.hotpepper.jp/strJ000032882/