Ba Be Lake & Cho Ra Town -- Dining is pretty minimal in this area. If you stay in the park, there's a canteen with basic, overpriced grub, but it's rarely busy, so it's kind of an ad hoc operation. You're on your own in Cho Ra Town, really, and any Vietnamese-language basics ("com trung" for rice and eggs, for example) or knack for charades will help out at the few little com (rice) and pho (noodle soup) stands along the main drag near the central market (at the corner where the road splits north to Cao Bang, west to Ba Be Lake, and south to Bac Can).
Cao Bang -- Dining is limited in Cao Bang. Go for "point-and-shoot"-style dining at the little local buffet at riverside, Huong Sen Restaurant. "Point" at what you want and say "Shoot!" when they charge you double for it. But even at double the price, it's still a bargain. They've got fried egg, tofu dishes, vegetables, fried river fish, and rice.
On the main drag, there are lots of good local coffee joints, the best of which is the Trung Nguyen Coffee Shop at town center. For a good, familiar snack, look for 89 Kim Dong St., where there is a good little stand selling banh mi (bread) and pâté. They'll make you a great little savory sandwich from just 8,000 VND.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.