If you do not have much time to look around, a good place to have dinner is at the street stalls next to the Ben Thanh Market. There are several restaurants one next to the other. As always, choose the most crowded one. Usually people know what they do.
I really enjoyed the food in these restaurants. You can come here several days, and taste a different local dish every day.
By the way, I almost forgot, bring some mosquito repellent!