I stayed for 6 months in Tianjin and, being honest, I wouldn’t recommend even a day trip. There is an old town which is not particularly well done, a river walk which is ok, a viewing platform on the radio tower which has good views, and some districts which demonstrate its cosmopolitan past.
But overall it is just one of the giant Chinese cities, often polluted and without any clear reason to visit. Should you be considering a day trip from Beijing, I would say prioritise your time for Beijing itself or the surrounding Hebei province rather than Tianjin. And for tips of the gems to be found in Beijing, see Beijing highlights for 2 days – 10 tips.