I guess it depends on the traveler's circumstances. If staying with family or friends, it would be pretty straightforward. If not, one possible plan - just off the top of my head - would be to arrange accommodations for a place that allows no-fee cancelations as late as possible, with your potential stay starting on the day you are supposed to depart for the States. US policy is that you need a negative test three days prior to the day of departure, which effectively means four days before the day of departure. The CDC webpage gives the example that a person due to depart on a Friday can take his/her test as early as the Tuesday before. Take the test as early as possible. If you test negative, cancel the hotel reservations. If positive, then you'll be staying a while longer. As for groceries and supplies, I'm not sure.