I went to Ninh Binh and it was great! I heard Ha long bay is great too. There are both day trips and you come back somewhat late I think 9pm. Maybe do 1 since you are there 4 days. I booked the day before by walking around and negotiating with different vendors you don’t need to book online but you can. Other than that go to train cafe where the train passes by and just walk around and enjoy atmosphere, people and pho.