You will not take a European vacation without entering a church. I doubt it is even possible. So, here are a few guidelines to follow when entering places of worship.
Dress: Usually in Catholic churches, no one is going to kick you out if you are not dressed properly. That doesn’t mean it’s not offensive when you don’t dress the part. Orthodox Temples are more likely to deny admittance for improper dress. Either way, it’s best to be respectful. When you enter make sure to remove your hat. In Orthodox buildings women are required to cover their heads with a scarf so it’s a good idea to pack a shawl or a light weight piece of fabric in your bag. Synagogues require that men and boys wear a Kippah but if it is required of visitors they are usually provided. It is considered rude to expose your shoulders so avoid tank tops or bring a cover. You should also wear pants that at least cover your knees. Try to wear soft soled shoes. The acoustics in these buildings are incredible and the wrong shoes can serve as loud and unwelcome distraction.
Taking Photos: Most of the time it’s ok to take pictures but make sure to check for posted signs just in case. Even though it is usually alright to take pictures there are a few rules to follow. The majority of places I’ve been ask you to turn off the flash. Also, never take pictures during Mass or during any service. Don’t gather a large group of people in front of the altar or next to a Saint and say “cheese”. Try to stay out-of-the-way and be respectful. Just remember that it was place of worship before it is was tourist destination.
Food: Spit out your chewing gum and finish your snacks before you enter.
Whisper: Use reverent and hushed tones. Do your best to keep your kids quiet (I know it’s not always possible). If the little ones are really riled up it’s a good idea to come back when they are feeling a little calmer. Maybe take a lap around and see the outside first. It’s also good to remember that Jesus Christ said “suffer the little children… to come unto me” (Matt. 19:14) So, don’t worry too much if they aren’t perfect angels.
A couple more things: 1) It is considered rude to enter the altar area or cross directly in front of it. 2) If you do not belong to the religion, you don’t have to cross yourself, light a candle or participate (though you are usually welcome to do so). It is just fine to pass through as a spectator.