Check in, Check out

Hotels
What do you do when your hotel check-in time is well after your arrival into a city?

• Large hotels often allow early check-in with no extra fees if they have clean rooms available when you arrive
• The hotel may allow you to leave your baggage in a storage room if they have one
• You may rent a locker at the train station

What do you do when your hotel check-out time is before wahen you plan to leave the city?  See points two and three above.  That way you have more time to sight-see before heading to the next city or to the airport.

Planes
American airlines tend to open online check-in 24 hours before the flight.  I noticed Air France allows check-in 30 hours before the flight, which is a wider window for you to try to check in early and nab a seat if you have not purchased a seat reservation.  I have never purchased a seat reservation and have not had issues traveling off-season to get the type of seat I want (i.e. window vs. aisle).