Y'all, For August 2025
Day 1 arriving FRA around noon, renting a car and drive to Eltz, staying in a hotel near Eltz
Day 2 checkout and drive to Frankenstein, checking in a hotel near Frankenstein or near Frankfurt airport
Day 3 - checkout Frankfurt before a flight to Prague around 4pm

Questions
1) Does the itinerary look reasonable? Can I squeeze in another location to visit on Day 2?
2) Never rent a car in Europe before. I assume driver's side is the same as in US, correct? Anything I need to know to ask at the rental desk (insurance, tolls etc, fill the gas tank when return etc..)
3) My flight to PRG is 16:50. I assume boarding is about 16:00. What time should I be at the airport (incl security check etc..).
4) My trip include 3 locations (Frankfurt, Prague then Istanbul_. I heard I need some type of registration required in advance. Do I need only 1 or 3 for 3 countries? How long does it take for each approval? Website to register please.
Thanks all.

4. No idea what registration is
Car hire terms and conditions are supplier specific
I don't know where you are coming from but driving after crossing the Atlantic is a bad idea.
Traffic rules are country specific
Yes you could visit other places on the same day. August is a holiday month with many Germans away and more just on holiday in country. Booking hotel rooms now
Boarding times are airport specific 50 minutes looks short but will be on airport website. It will depend on baggage and airline. Travel light, auto check in etc will speed up the process.

Where are you flying from?
Driving after a long haul flight is downright dangerous, to you and to all other road users.
Driving after a long haul flight when you have never driven in Europe and do not know the laws or roads is absolute madness.
Why the rush to see these two castles?
Burg Eltz is a good hour and a half, and maybe longer depending on road conditions, so by the time you have exited the airport, collected your car and driven to Burg Eltz it may be close to closing time.
Burg Frankenstein is a couple of hours from Burg Eltz, in theory.
Honestly if you are desperate to see them both, and nothing else, then stay in Frankfurt when you arrive then rent a car and do them both the next day. German autobahn are not for the tired or fainthearted, and nor are German towns and villages with roundabouts, random diversions etc etc.

Boarding will be announced at the airport. The flights close early to avoid fines. You need to be at the airport in time to get through security and find your gate. Do not leave it to the last minute, and remember returning your car and getting to the terminal take time. You should be able to check-in online and have your boarding pass on your phone.

Don't forget to check you can use your phone in Europe without running up huge extra fees,

Can you drive a manual, stick gear box? You may or may not get an automatic. The T&C of the hire company will tell you what insurance is included and what you need to buy or use your credit card for.

You need to check whether you need an e-visa for Turkey. You can find that info online.

Don't forget to check if you health insurance covers you when abroad, If not you need to get travel insurance.

You may need to get an ETIAS (European electronic visa waiver system). It was supposed to be rolled out earlier this year but everyone is still waiting. It seems unlikely at this point but do keep an eye out for that.

You don't need a car to visit Burg Eltz. You can catch a train to Moselkern and from the station you can walk (30 mins) or catch a taxi (10 mins) up the mountain. Burg Frankenstein is being renovated and will be closed sometimes. They have closed the restaurant there. You can get there by public transport but it is a bit harder. TBH I would prefer just to see the Mosel and the middle Rhine gorge, that would be enough for me.

Lavandula

ETIAS isn't coming for some time. They are rolling out EES first spread over a few months. Once that is working ETIAS will begin.

https://travel-europe.europa.eu/revi...-2025-03-07_en

Thanks for that hetismij, I didn't know when it was starting (and still don't ). I am watching myself as my husband and daughter will be in Europe in April and I am still the chief organiser even if not travelling myself. I will be happy if they (the EU) just delay it to May.

To the OP - do have a look at Deutsche Bahn. Although there are complaints about its lateness nowadays it still outstrips most other rail systems. There are slow trains up and down the Rhine and Mosel, and you can buy tickets that will let you hop on and off all day.

Lavandula