Less than a week?

Are all rentals a week minimum?

No. People who have "Points Only" timeshare contracts (as opposed to having deeded ownership of a unit / week) can reserve partial weeks. Such "Points Only" contracts exist with Wyndham, HICV and other "chains". However, it is not very common to see rental listings posted for partial weeks.

Actually finding a partial week rental advertised and available in the location (and during the dates that you wish to travel there) is the dificult part.

Owners of deeded full weeks will only rarely entertain renting out partial weeks. Such owners must pay maintenance fees on full weeks, whether their unit / week is used for a full week, used for only a partial week or just sits unused; full maintenenace fee for their owned week applies, regardless.