Baan Esan, a traditional Thai izakaya, is a favorite with the locals. It's conveniently located a stone-throw away from the South Exit of the Koenji Station. You can get excellent Thai dishes at prices that are very kind to the wallet. The excellent green curry, for instance, is only 600 yen. Most orders are about that price. Many people order a couple of food items and share among friends. There is a large selection of drinks, all for about 500 yen.
Curry Bal Kujira
Curry Bal Kujira is a small resto-bar that specializes in curry. It has a popular dish for which you can choose two types of curry (1,200 yen). Among the choices are chicken, coconut, pork, and beef. The spices are just right, and there is a little acidity that gives an extra dimension to the curry. At night, people tend to order small side dishes while they drink and finish with the curry. The draft beer is only 350 yen! This is a really nice quiet resto-bar located in an area of town that gets a bit rowdy at times.
On a hot summer evening in Tokyo, few things feel better than having cold drinks in an Okinawan izakaya. Urizun offers a great laid-back, Okinawa-style atmosphere, and cheap traditional foods of the island. There is a wide choice of side dishes and some more filling plates as well. Try the goya champuru (730 yen) if you're not too familiar with the cuisine of Okinawa! It's a kind of goya (bitter melon) stir-fry. Drinks are about 500 yen. They, of course, have Okinawa's famous Orion beer!
If you've never experienced doing "tachi nomi" (drinking at a standing bar), this might be a great place to start! The place is really well-liked by the locals. It has a friendly atmosphere and serves delicious accompaniments to their drinks. You can have sashimi, grilled fish or pickled vegetables. A good thing about standing bars is that everything is so cheap! Most sashimi dishes are only around 300 yen, and so are the drinks! There aren't anywhere for you to sit down at Nanasuke, but on the other hand, it makes interacting with the other customers easier.
Last but not least on the list is Beer Kobo. There, you can have freshly-brewed, in-house delicious craft beer. Prices are reasonable for the quality (about 500 yen)! English menus are available. As side dishes, you can order French fries or other deep-fried snacks to munch on. What you should try the most are perhaps the sausages that come with their own small burner so that you can grill them yourself as you sip on your beer!
If you like Koenji, please check out the link below! It has some other suggestions about where to eat and drink!