Like the US, Europe operates on hub & spoke flight system. If you’re flying between two hubs, flying might be the longest travel option.
Last year I rented a car to drive from Eastern Germany to Krakow, Poland. A couple of things to thing about with car rental.
[li]Most European cars are stick shift. You can get an automatic but you’ll probably have to prebook it if you don’t drive a stick.[/li][li]Depending upon your US mobile carrier & where you are, you might not have coverage, or it might get throttled down to 3G. For a few USD/EUR/PLN a day the car rental place will rent you a Euro Garmin (or other GPS) unit. Since your American unit probably doesn’t have Euro maps loaded this is well worth the nominal investment, especially when you’re dealing in road signs in a language that you don’t understand.[/li][li]US car insurance follows the driver, German car insurance follows the vehicle. I called my credit card company before I left & they assured me that I could decline the optional insurance abroad. However, what I didn’t find out until I got there was I needed a (personalized) letter talking about the coverage I had thru my credit card. Since I was trying to rent in the morning in Germany, the specialized US office wasn’t open yet so I had to buy the rental insurance for one day until I could get this straightened out.[/li][/ul]
Some rental apartments &/or hotels have parking included so you can plan on leaving your car parked & take public transit; however, having the car allowed me to go somewhere I couldn’t get on public transit & would have had difficulty getting a taxi back but was absolutely the highlight of my trip.
As for crossing international borders within the EU, if you’re on a common carrier (plane, train, bus) you won’t even know when you crossed the border. In a car, I usually see larger/more signage saying, “Welcome to <State>” than I did at the Polish / German border.
Many/most of the public toilets are pay toilets, the equivalent of a US dime or quarter, but you will need some small local currency.
Feel free to PM me for some specific site suggestions in those countries.