Work backwards from airline guidance
Use your airline's current check-in and airport-arrival instructions for the actual flight. Add the journey to the airport with a margin appropriate to your route. Don't use a general city estimate as a guarantee.
Keep that departure from your final stop fixed while the outing remains optional.
Resolve luggage and checkout first
Ask the accommodation about storage or late checkout and confirm any charge. Avoid planning a walk or museum visit with luggage unless the venue has agreed suitable storage.
Keep travel documents and items needed for the flight with you rather than in a bag someone else is collecting.
Choose a small nearby activity
A meal or short visit can fit more easily than an excursion that depends on a distant return transfer. Set a time at which you will drop the activity if the day changes.
Put the airport journey into your plan before the optional outing. Confirm the pickup address and driver contact, and resist adding a final stop simply because there appears to be an empty hour.
Rules, fees and opening hours change. Confirm anything that matters before you rely on it.