Choose what you most want to keep
A flexible outing has a clear priority. Pick the one visit that makes the day worthwhile, then label everything else optional. This is more useful than a full schedule in which every stop appears equally important.
If a late start removes an hour, you should already know what to drop. The group shouldn't have to negotiate the whole day from a parked car.
Keep the alternative close
A fallback meal or indoor stop should fit the area you're already visiting. Replacing one cancelled activity with a long cross-city journey may create a second problem. Check whether your alternative needs a booking before relying on it.
For an Accra heritage day, the National Museum might be worth investigating as a different focus, subject to its own opening arrangements.
Send one updated plan
When the decision changes, send a single message with the new meeting point, time and cost implications. State which previous instruction it replaces. Multiple partial updates make it easy for someone to follow the wrong version.
Treat a shorter outing as a valid result. A good meal and one thoughtful visit can still be a full day when the alternative is rushing everyone through a plan that no longer fits.