Cave on Island, Ha Long Bay
Ha Long Bay is on the coast, east of Hanoi and a little north of Haiphong. In the town of Ha Long, there are many boats that can take you to one of the larger islands in the bay, on which there's a fine limestone cave. It's lighted and a good trail goes through it.
The lighting of caves in Vietnam is a little bit different from, say, Carlsbad Cavern in the U.S. It's well done and tasteful, but some of the lights are colored. Also, you need to watch your footing on trails inside caves. You may discover unexpected, small steps in the semi-dark and you want to handle them well, not poorly.
It takes probably an hour to reach the island and another hour to tour the cave. Then back to Ha Long again. The town of Ha Long itself isn't special, and is said to be somewhat dangerous to walk around in after dark. There are nice hotels, and these are relatively cheap in Vietnam.