Dong Hoi is the closest town from where you can visit the impressive Phong Nha Cave. Although there is little else to see in the area, there are some nice beaches that can be visited on the other side of the river.
The main attraction of Dong Hoi is a trip to the Phong Nha Cave. This spectacular cave was used by the Northern Vietnamese as a field hospital during the war and it has the scars to prove it. The front face is pockmarked from attempts to lob bombs into the entrance by US helicopters, in the hope of collapsing the entrance. Fortunately their attempts were all unsuccessful as the cave makes for a fascinating visit.
Phong Nha cave is explored mainly by boat, though there are a couple of raised areas where your guide will walk you around and explain the various points of interest. The cave was not officially surveyed until 1990, however, the cave walls show evidence of it being a popular place to visit for quite some time.
The tour consists of a boat trip to the cave entrance taking around 45 minutes, then an hour drifting through the cave guided by a couple of gas lanterns. It is an eerie feeling as all you can hear is the gas lanterns hissing away, and the water dripping as you slowly drift from chamber to chamber.
The cave is over 7km long but the tour only visits the first 800m or so. There have been some enterprising travelers who have managed to bargain a few more hundred meters for the guide, and it is apparently well worth the expense.