Some random thoughts from my 48 hours in Paris a couple of summers ago and 5 days in Ireland last summer.
We had the $10 / day per device thing with AT&T. We just turned off cell data and disabled roaming on my wife’s phone so she stuck with wifi. My phone got the same plan as in the US for the $10 / day. T-Mobile has annouced something similar. My cost is $5/device/day but I may have a special deal and public plans might cost more.
If you think you might want to use a hair dryer in the UK or Ireland take an extension cord. It is against the electrical codes to have more than a shaver outlet in a bathroom and outlets are not as frequent on the walls as in the US. We still had a few times with a 6’ zip extension cord where it was suspended in the air for my wife to use a hair dryer in front of a mirror.
Take a few of those 2 wire plugs that allow you to plug in 3 more things. Several times in a hotel they allows us to extract the power form a small well with a flip lid on the hotel room desk.
As to the 120/240v issue we take an adapter for the countries and plug a US power strip into it. And only take power devices which can use either voltage. So one adapter (and a spare just in case) plus the power strip and we get a row of power plugs for our US plugs. I’ve since found a compact small strip with an 8’ cord at Target for our next trip.
Take a 10,000 mha battery. I’ve got one that is almost exactly the same size as my iPhone X. I use it on the plane when if there isn’t working electrical power. (My seat row wasn’t working going across the Atlantic on a new 787. Go figure.) This lets you get off the plane with a charged phone. And it’s easy to carry around during the day in case you run your phone down. Typically you can fit 10,000 mha into a pants pocket.
While many cards will give you Comp/Collision coverage in the US many do not overseas. But a few premium cards will give you Liability and maybe Comp/Coll. Check out your card portfolio and mabye avoid $100 to $400 for insurance for a week rental. Remember that we think of as a compact car in the US they think is a standard in much of the world.
As Adam noted, make sure have credit/debit cards with no extra fees for foreign exchange. And you can search for web sites to find out the conversion rates of various major bank CCs.
Google Maps will allow you to cache areas on their iOS app so it only fetches data for things that have changed since you loaded the cache. Plus it means you can have a working map if no cell coverage. So the day before we left for Ireland I cached the entire island.
Ireland has cross country buses that are basically the same as flying but more comfortable. Free wifi, great seats, etc… 20euro coast to coast. Departing from the major cities every hour or two.
Paris had a fantastic subway system. And they do their best to hide it. Seriously. It must be amusing to the locals to watch the tourists searching up and down the street for the unmarked entrance. Oh, and heaven forbid they tell you that no you can’t buy a ticket on this entrance. No go back up to the street and find the one for the other side of the track.
Roads in Ireland are wide enough for 2 of their standard cars to pass. Maybe. Meeting a tour bus is, ah, interesting. 3 times we were hitting leaves from the shrubbery on the side of the road. All the while knowing that 4 inches past the shrubbery was a wall of rock.
If you know your schedule, check to see if you can buy tickets in advance. We spend 4 hours in line for the Eiffel Tower as we were not sure of our schedule.
Changing the subject a bit, if you can handl a $1000 or few per month in credit card spend and have a decent credit rating you can get points for airfare and hotel stays before you go. 3 of our 5 nights in Ireland were paid via points. Google is your friend.