The weather is very hot and the sun burns. For the weather, I dont not suggest you visit the Great Wall, unless you come across a cool day. Forbidden City is too big and if you want to see it carefully, prepare a whole day, well it is closes at 5pm. Rent a machine guide. It is useful. English version is available there. Temple of Heaven is a must. You can visit Yonghe Lama Temple either. Beijing South Railway Station has bullet trains to Tianjin. No need to buy ticket in advance, just buy at the moment when you get to the railway station. It will get to Tianjin Railway Station in half a hour.