Start with fixed bookings like flights, train departures, and check-in windows. Then group activities by area. A museum and lunch within walking distance usually make a better pair than two stops across town.
Add transit and waiting time between stops. Leave one open block each day, and write down a backup for any activity that depends on the weather. Keep reservation numbers and addresses together so you can find them quickly.
The GGC Studios Trip Planning Workbook has separate pages for your itinerary, bookings, and packing list.
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