Rowboats bob along the Sao Khe River through limestone caves. It's a relaxing trip, but many caves have also been enlarged to accommodate boats. Boat trips (200,000d per person, or 800,000d for your own boat) take two hours, and there are two possible routes, both visiting caves and temples. Bring a hat and sunscreen as the boats lack shade. Trang An is 7km west of Ninh Binh. You'll pass it on the way to the Chua Bai Dinh.
A trip out here makes for a fun bike or scooter journey from town.