1. PLANNING: I highly recommend planning out your trip, day by day, ahead of time. Be sure to list out the big things on paper first. For example, you might want to see Windsor Castle and Stonehenge when in London, or visit the Louvre while in Paris. These things usually take up a whole day, so plan around the big activities first.
2. HIGH LEVEL OVERVIEW: It's time to figure out what dates you will be traveling from city to city, how long travel will take, etc.. For most of Europe, I recommend traveling by rail. There is a high speed train (Eurostar) from London to Paris and back that only takes about 2 hours. Other trains can be booked in advance on apps such as Rail Europe. Booking and paying for these up front saves a lot of time, and sometimes money as you can find the trains with the lowest cost. Look at the web sites and find rail times and trip duration to figure out when and where you need to be.
3. BOOK AIRLINES FIRST: You can certainly book all of your flights, hotels, rental cars, tours, etc. on various apps, but I highly recommend putting all of that info on one app. That way, all of your information is in one convenient location. Look at the various apps like Travelocity that show different flights and costs. You may find that the cost may change a lot if you leave on a certain day. Try a few different dates and see what the prices are. If you are stuck to dates that you can't change, then your only option is to try out different airlines. You can do connecting flights, but remember that changing planes can sometimes be a hassle. I remember running from one end of an airport to another to catch my connecting flight. Not something you want to do especially if your carry-on is heavy. However, you can save money by connecting. These are all your standard airline questions you will need to work out. For me, I prefer a direct flight with no connecting airports. It's best to chose one site that has an app, and you can possible bundle flights with hotels and rental cars.
4. BOOK HOTELS: Look for hotels, then book and pay for them in advance. You can save money by paying for your hotels in advance, and there is no issues when you go to check-in. Many people think they can just walk into a hotel and get a room, but I witnessed many people trying to get rooms and heard the same "sold out" response that made me glad I chose to book in advance. In addition, the cheaper option now is to make it non-changeable or non-refundable. If you know you are going, and won't change your mind, this is the way to go. Just remember that if you can't make the trip due to illness, or other reasons, you lose that money if it's non-changeable and you can't make it.
5. BOOK TRAINS: Book your trains next. Save those tickets in your iPhone wallet or other convenient place. Paper tickets are rarely used now, so having everything on your phone works well.
6. BOOK CAR RENTAL: Note that most of Europe drives on the right side, just like the US. You drive on the left only in the UK. If you plan on driving a car in Europe, search ahead of time. There are the standard rental companies like Hertz and Avis. When in France, I used Sixt. The biggest problem with car rentals is figuring out where to pick up and drop off the car. Looking on their website, you will notice dozens of pick up and drop off locations. I finally just booked the car from the CDG airport in Paris, and had to take a 30 min cab ride there. You can try to find rental cars once you arrive, but you will be at the mercy of what cars they have available.
7. BOOK TOURS: We went on the Windsor Castle/Stonehenge tour when in London, and booked that well in advance. These types of bus tours fill up quickly, so you may not get a chance to do them if you wait until you arrive. Also, by pre-booking, you might get a discount.
8. GATHER DOCUMENTATION: You may require more documentation that this, but at the bare minimum, take the following:
PASSPORT - This may seem obvious, but be sure you have an up-to date passport, and make sure it does not expire until after you return to the United States.
VACCINATION CARD - Each member of your party should have a vaccination card including all the recommended vaccinations and boosters. Not all countries require it, but France is one that currently does.
9. DOWNLOAD APPS AND BOOKMARK WEBSITES: Here are my recommendations:
TRAVELOCITY - I used the Travelocity website to book my airfare, hotels, and tours. This made it convenient for me to use the iPhone app to check which hotel we were staying at on what date, etc. On our trip, we stayed in five different hotels, so keeping track of where the names were with addresses all in one spot helped when we needed to grab a cab and tell the driver where to go. If these were in notes, or on different apps or sites, trying to find them when you need it would be tough. There are others, but for our needs, Travelocity worked well.
EUROSTAR - If you want to get from London to Paris (and back) quickly, this is the high speed train linking the two cities. Download the app or at least add the tickets to your iPhone wallet so they are ready when you need them.
RAIL EUROPE - I used this for taking the train from London to Durham. It has many connections between cities, and is a fast and convenient way to travel safely and comfortably.
TRANSLATION APPS - You don't need it, save your money. We don't speak French or German, but had no problems even in the most rural areas. The one time I needed to tell a French parking lot attendant, who spoke zero English, that we would be back in an hour after eating at a café, I used Google translate and so did he. In our travels, 90% of Europeans speak English in the cities, and most gas stations or stores outside of cities are self-service and use contactless payment cards. In fact, we did try to use a translation app, but they need the internet to translate. When you are out in a small German or French town where it would be useful, you can't get service to use the app! In my opinion, they are all junk.
10. PURCHACE ADAPTERS: You will need adapters and converters when going to the UK and the EU. Note that the adapters are different between the UK and EU! The one that worked best for me was the BESTEK Universal Travel Adapter. It has both types of plug, and converts to 110v. It also has 4 USB ports for charging phones and tablets. I also used a standard power strip with it and it worked well. The only other adapter I used was the Unidapt Universal Travel Adapter for higher powered items like a curling iron or hair dryer. Most hotels have hair dryers and irons that use that country's plug already, so don't feel the need to take a dryer or iron. When travelling by Rail, the Unidapt worked great, but it is rather large. If you can find a smaller plug, feel free to take it instead.
After all of this, you should be well prepared for you trip to Europe!