Place buffer time after a long move, before an international departure, or between weather-dependent experiences. It gives the route resilience without making it empty.
Use it as a soft day first
Plan a neighbourhood, market, or optional museum rather than a blank hotel day. Then it still feels valuable if nothing goes wrong.
Let it absorb the useful surprises
A restaurant recommendation, an exhibition, or a slower morning can become part of the trip because you did not schedule every hour in advance.