I make that an 11.5 hour layover.
First decide where you want to be at dusk. Up high is a memorable experience at this time of the day, so you'd head for Shinjuku to be at the top of the Tokyo government metropolitan tower to see the city change from day to night (and perhaps catch Mt Fuji) or you might like to see the neon switch on closer and be walking around Ginza this time. From then until you have to head back, it's compulsory 'play' time for many workers so the bars and izakaya districts will be heaving. You could easily stay in Shinjuku or spread out from Ginza to Shimbashi or Tokyo station or you could head to Shibuya if you like to hang around younger people or Roppongi if you want to find lots of people to interact with in English.
Alternatively you could try a public bath or spa, there's a whole thread on Tokyo Onsen in the forum. I only know the ladies' sides but they are generally very social so a natural place to interact with non-drunk locals.