This classic Japanese small apartment is great for a solo traveller or a couple. All necessary amenities are included. Pocket wifi (better than mine), washing machine, microwave etc...
You will be on the edge of Shinjuku but in a quiet residential area with local restaurants and even a supermarket nearby.
Plenty of transport options whether it's 5 mins walk for bus (to Shinjuku / Shibuya), or Trolleycar (to Ikebukuro) or main train / metro lines 15 mins walk away.