Thank you, Tom R. This is helpful since this is absolutely new for me.
I will be travelling (driving) solo on a portion of the trip and feel
certain that I will need GPS. Verizon fee is $10/day for travel pass
and I usually access this, but it could add up on this trip.
One tip related to your comment about GPS is that you can - while still in the US or on wifi at a hotel - "download" google maps for an area(s) to your phone. Most smartphones have GPS built in (no data plan required) but that just gets the gps coordinates, so you need a downloaded map (or your normal connection to the internet) for those coordinates to be plotted on a map. By downloading a map or series of maps where you will be traveling (eg: downtown Paris), you can navigate a city/location "offline". It won't be able to do smart routing - ie directions from A-B - but you will at least be able to see on the phone's google maps where exactly you are.
This is NOT my preferred way to use GPS (I usually get a data plan), but it is a great back-up/safety option (sort of like a paper map) and also a "cheap" way to avoid any data plan if generally staying in a relatively "fixed area" like a city. IOW, you probably don't "need" a data plan if just staying in Paris/Amsterdam/Madrid/etc., as you'll be on and off wifi at the hotel, museums, cafes, etc., so downloaded maps give a phone a cheap alternative if you realize you don't need the whole nine yards of a data plan.