Beijing to Tianjin

Although your time is tight, it is possible to cover all the three spots in one full day. From Beijing South railway station, there are bullet trains to TJ, and the travel time is about 30 mins. The first train departs at 06:13, and the last bullet returns at 22:53, so you have plenty of time for sightseeing.
I suggest take the bullet train stopping at TJ Railway Station.

Upon arrival, take the subway line 2 from the railway station to Dongnanjiao Station. Take the Exit D and walk about 860 yards, you will reach the ancient culture street. (suggested sightseeing time: 1.5 - 2 hours)

After the sightseeing, walk to Dongbeijiao bus stop to take the bus No. 1, 619, 646, 670, 609, 671, 610 or 904 to Jingangqiao Station. Then walk about 660 yards, you will reach the Buddhist temple. (Suggested sightseeing time: 2 hours)

Then walk back to Jingangqiao Station to take the bus No. 2, 611, 878 to the Nanshi Food Street. (suggested sightseeing time: 2 hours)

Afterwards, walk about 500 yards from the food street to Rongye Street to take the bus No. 905, 639, 676 or 635 to the Haihe Square of the railway station directly. Then take a train back to Beijing.