Between the DMZ and transport to the Lao border, a lot of travellers end up spending a day or two in Dong Ha, and oh, brother, is there nothing to do.
Food-wise, there's a string of interchangeable restaurants on the corner of Le Van Huu and Le Duan, each with its own hawker out front frantically trying to wave passing vehicles into their place. They all serve the same menu, and the food is edible and cheap, but if you arrive when a bus shows up you may have to wait forever to get served.
Otherwise, all we can do is recommend you get on a xe-om and ask them to take you to Bia Tuoi Dong Ha on Huyen Tran Cong Chua (you might need to write that down on a piece of paper. We'll wait...) On foot, heading from the DMZ Guesthouse past the Mekong Hotel, take the first big left and then your first right -- it's on the left. There you'll find a bar/restaurant with a pleasant, quiet courtyard out front. Draught beer is 7,000 dong per litre, and the menu features a delightfully perilous selection of Vietnamese dishes -- all in Vietnamese only. Close your eyes and point and see what happens. It's the most excitement you're gonna get in Dong Ha.