Give ideas their own place outside the committed schedule
An optional list can contain a museum, a park walk and a visit to friends without making anyone available for all three. Add one line beside each idea that makes choosing easier: opening hours to check, a suitable weather window, or a friend to ask. Keep the list light. Its purpose is to make a free afternoon easier to use, not to create a second set of obligations that also needs maintaining.
Recognize the step that turns an idea into a promise
Check the optional list when a suitable gap appears. If the museum requires tickets, make a real decision before buying them. Once money is paid or another person is counting on the visit, move it to the confirmed calendar and name the organizer.
Check whether a tentative suggestion has participants
Someone might say “we could go to the museum Saturday,” while another person hears that as a confirmed outing. Ask whether anyone agreed to the time, whether tickets have been bought, and whether a host or helper is counting on the visit. If none of those conditions applies, return it to the ideas list. If there is a real commitment, name the organizer and record the date, cost and next action in the calendar.