In Paris, the popular Notre-Dame Cathedral opens at 8 a.m., which Ms. Gacheny said is the best time to go. To escape the afternoon heat, focus on museums like the Musée d’Art Moderne de Paris, or the Musée Carnavalet, where you can hear about the history of Paris for six hours in an air-conditioned aristocratic mansion. In extreme heat, make sure to check the museums’ websites beforehand, since they sometimes close early because of high temperatures.
In London, the upper floors of the Victoria and Albert Museum are Ms. Wignall’s personal favorites. Historic churches are also a great place to cool off, she said, pointing to St. Bartholomew the Great and St. Paul’s Cathedral, both of which are generally less crowded than the British Museum or Westminster Abbey.
Start walking tours early, and bike in the afternoon
In France, with the heat soaring during the day and overnight temperatures hitting records, Ms. Gacheny said it’s generally good advice to start early.
If you want to discover Parisian neighborhoods, walk around from 7 a.m. to 11 a.m. Take an afternoon siesta at your accommodation, or grab a bite at hotel bars and cafes, which are the best places to find air-conditioning, Ms. Gacheny said. Then head back out around 6 p.m. for a breezy bike ride around the city.