I found a juice box next to the sake isle at a local conbini. Thought it was strange until I read the box to realize it was sake (日本酒/にほんしゅ/nihonshu) and not juice.
Just take the straw off the side and punch into into the foil cover on top to have a convient drink on a nice spring day. The tiny boxes are very cheap, but not very good quailty that you would be used to from a bottle of sake. There are several kinds of juice box sakes, but I have found the 鬼ごろし/Onigoroshi to be the best tasting one.
Perfect for blending in with my kids at the local neighbourhood park.
If it is nicer weather in Tokyo, you might also find this Aooni IPA which paired nicely with the sake and was just what I needed before grilling up some dinner for the family.