What should the crew and passengers do on a long voyage?
If they do nothing, their attention is scattered and may become remiss. You can begin to wash the boat, but I do not think that is the best solution for a vacation.
What are suggested best practices?
One obvious activity that will appeal to many (but not all) outdoors people is watching the local wildlife. The sort of voyage in the question can't be far from shore, so watching for birds and cetaceans is a good way to spend time on deck.
Ideally you'd have field guides (which may also be interesting reading) and binoculars (not necessarily a pair for every person, but not spread too thin). A more knowledgeable member of the crew can often make this more interesting for the others by interpreting the field guide, and checklists may appeal as well.
At night, you're likely to be away from light pollution so at least in calm conditions there's a lot to see in the night sky. A star chart (possibly as an app that doesn't need data) and those same binoculars are all you really need.
Some people may also be interested in weather observations and natural navigation cues. Another couple of books will go a long way there.
These will never please everyone, because they all involve looking out from the boat at the natural world, but if the voyage is going well that's what many people will want.
This very much depends on what sort of voyage it is.
Learning the skills required to handle the boat would be a classic. Specifically learning navigation and standing watch.
We used to spend a lot of time playing cards and doing decorative knotting as teenagers on longer voyages.
If you're on a small crew vessel, scrubbing the decks doesn't take much time but there's always cooking and cleaning to do, along with minor repairs and maintenance. Wooden sailing boats especially need fairly steady maintenance.
You can fish if you have the personality for it, but a good book is always a favourite.
