Exploration does not need to become an expedition to be worthwhile. A map, a spare hour and one unfamiliar turn can be enough, provided the practical limits are understood and the place is still allowed to surprise you.
The question behind plan an outdoor day without overplanning it is what changes in practice. The useful answer is usually found between set the non-negotiables and review the useful parts afterwards: the point where a broad idea becomes a choice that can be made, observed and revised.
Know the access rules, likely weather, daylight, water, communication limits and the point where turning around becomes the sensible choice. These decisions create freedom later.
Define the main route or objective, then identify a shorter version and a safe exit. Alternatives are not failed plans; they are part of planning for a real environment.
Leave some choices for the day
Do not pre-decide every stop, pace and photograph. Conditions and interest should be allowed to change the details.
Record access changes, timing surprises, equipment that mattered and ideas for next time. Keep it practical. The aim is a better next outing, not a performance report.
Good preparation should create options rather than a script. The route can change, the weather can win and a shorter day can still be worthwhile when the purpose is curiosity rather than proving the original plan correct.
That is the standard I would use when returning to plan an outdoor day without overplanning it: keep the reason visible, make the next decision small enough to understand, and leave enough evidence to know whether the approach is still helping.