The dessert is called Anmitsu, and has been around for decades. It's got red azuki beans, jelly, fruit, and sometimes ice cream. You pour black sesame sugar over it to give it texture. Enjoy. When finished wash it all down with hot barley tea.
If you need a little more, then order the coffee. Japan's coffee culture is on the same level as Europe now and is no longer playing catch up. Trust me. Not to include espresso, though. Still have a ways to go.
Directions are really simple. Right near the Ryogoku Station near the main street. Lovely Jukujo are there to assist you.