I love coming to this square not only because the cathedral (the whole name is The Cathedral of the Lord’s Transfiguration of all the Guards, see Wikipedia for more) is so beautiful but also because this spot is so peaceful. You really feel the difference coming from busy Liteyniy or Kirochnaya.
I’m not religious at all but it’s very nice to walk around and hear a choir singing on Sunday inside the cathedral or to watch yellow leaves fly around in autumn. I find this cathedral to be one of the most beautiful in St. Petersburg but not so well-known among tourists and even citizens, therefore, I recommend you check it out
From here, you can head to Tauride Garden or Radishcheva Street to keep quiet for some more time. Or if you choose to eat something vegetarian instead, choose between Botanika and Auroville restaurants.
Right now the cathedral has been repainted from the outside and work continues on the inside, so you cannot enter, but you can have a look at it.