Thang Hen Lake
Surrounded by mountains, this serene lake is at its best in the rainy season. During the dry season the lake shrinks and is not as impressive, but there are a few rafts that you can take. Walking in the nearby mountains brings you past local herders and farmers.
Ban Gioc Waterfall
This breathtaking waterfall, the largest in Vietnam, straddles the China-Vietnam border. The water is particularly thunderous in the wet season but is still beautiful during the dry. You can take a raft out into the river to the base of the falls and greet Chinese tourists doing the same from the other side. Ban Gioc is about 85km from Cao Bang, about 3 hours away.
Nguom Ngao Cave
This immense cave holds a series of majestic, jaw-dropping caverns, filled with delicate, intricate formations of stalactites and stalagmites. In the low season, you may even have the cave to yourself, an experience probably not to be had elsewhere. Optional guides will helpfully point out particularly interesting rocks and bat roosts.
Pac Bo Cave