The school year gives a child a shape they did not have to think about. The summer holidays take that shape away all at once, for weeks.
Rather than planning each day, name a rhythm across the week — one outing day, one at-home craft day, one "free" day with no plan at all. A child can hold a weekly rhythm in their head far more easily than a shifting daily schedule.
A similar wake-up time, or the same breakfast routine, gives the day a familiar start even when everything after it has changed. Losing every single anchor at once is what makes the first week of summer feel so unmoored.
Not every hour needs an activity, and a summer scheduled as tightly as term time defeats the point of it. A completely open afternoon, planned as such, is different from an afternoon nobody got round to filling.
There is no fixed ratio — some children want more structure than others. Watch for the child asking "what are we doing today" with genuine distress; that is the sign to add a little more shape.
No, though it can help with both structure and childcare. Plenty of summers work well built entirely around a family's own rhythm.
Part of our guide to Filling the unstructured days.