We sailed around, oogling the jagged rock formations and glorious sunsets. On the first day, we anchored just off a hidden lagoon and spent the afternoon jumping off the roof into the warm blue waters and frolicking about. The other members of our boat thought Karen and I were strange when we each set off swimming for some much-needed exercise. It felt great, though.
In general it was a very relaxing trip, especially after the barely-controlled but exhilarating chaos of Hanoi. We also explored Cat Ba Island on bikes, and an immense cave system that went on forever, culminating in a giant cavern, complete with stalagtites and underground rivers, that could hold my high school's football stadium and then some.
If you are ever in Vietnam, I recommend a visit to Halong Bay. It a bit of a trek to get there, but worth it. It's also huge! You could sail for weeks and never see it all, if you were so inclined.