The logistics of your major side trips may change your plans, but this is usually a good thing.
Early on, you need to work out how long your major sub-trips will take.
Add a city on arrival
One year, we booked our main flights Victoria-London and Brussels-Victoria, then got on to the next level of planning. We decided to go to Brussels via Switzerland, and that spun off some interesting decisions. London-Zurich and London-Geneva aren’t very long flights, but what with getting to the airport in London and going through security the hours will zip by fast. From either Swiss city it would be at least three more hours to our destination in the Alps, and that’s just more travel than I wanted in one day. We didn’t much want to stay in expensive big-city Zurich or Geneva. So I did some more digging, and we spent that night in beautiful Lucerne before moving on to the Alps.
Break up a long ride
The problem is reversed as we leave the Alps. Going all the way to Brussels would be a full day of travel, no matter how we went. So I researched some more, and decided to end that day in Nancy, France after a three-hour stop in Basel, Switzerland on the way. This decision actually forced us to take a day off our planned time in England, but we had allowed for that in our high-level planning.
One one trip to Rome, we allocated a full day to take the train to Tivoli, to see Villa d’Este and Villa Hadriana. On another trip there, we gave half a day to the Aqueducts and a separate half-day to Ostia Antica. In both cases, of course, we had to work out how to get there, how long it would take to get there and back, and how long we’d be there. PLUS where to have lunch, and where to go when we returned to the city. In both cases, the return train took us right to our next planned sight. That was NOT a coincidence.
At a simpler level, if you want to see a particular concert in city A, you need to find out its times. If the show ends at 10 p.m., you probably don’t want to be staying 40 miles away in city B. But staying on city A might wreck your plan to be in city B for sunrise.