Itinerary Greece, Croatia and Spain

Consider cruises that stop in the Greek islands that you want to see as well as Athens. Some cruises that include Greece also include the Croatian cities of Dubrovnik and Split.

Two months is a lot of time for those places. Santorini and Mykonos would be worth a couple of days each, while you could easily spend a week on Crete. I haven't been to Rhodes, but that might be another to visit.

Getting around Croatia will be more difficult, the road are ok, but not like Germany or Italy. Dubrovnik is not close to anything.

Regarding Spain, from Valencia, you could visit Grenada, Malaga, Seville, Cordoba, Madrid, Toledo, Segovia and if time Barcelona.
You will likely need to fly from Greece or Croatia to Spain.

Consider an open jaw ticket, flying into one city and out of another.

Thanks. I've been to all the places in Spain you mentioned. So I'd most likely spend a couple of weeks in Valencia on the return and then fly back to the US from there. An alternative itinerary might include Valencia, the north of Spain, and then working my way into Portugal. I've never been to Portugal. Geographically that might make more sense. Then doing Greece, Croatia and Lubljana another time. Thoughts anyone else?

You can start by exploring transportation links on Rome2Rio. (Do not, however, trust its time or fare estimates.) Greece to Montenegro to Croatia will mean buses if you do not fly, and flying is almost certainly going to introduce you to airports not on this trip's wish list (hello, Vienna? Budapest? Munich?). Check skyscanner for flights, being aware that flights may be day-specific.

When Rome2Rio suggests a bus option, keep drilling down till you reach the name of the bus company. Usually there will be a link. If you dont' find one, try Googling "bus Kotor to Dubrovnik), etc. For buses to, from or within Croatia, the BusCroatia website seems helpful. I haven't been to Greece recently, but the buses in Montenegro, Croatia and Spain were quite comfortable.

Border-crossing buses are infrequent, so pay particular attention to their schedules, and double-check those schedules as soon as you get to the departure city. Do not wait until departure time to buy a ticket on an international bus.

Suggested destinations in Montenegro: Cetinje, Ulcinj, Budva (similar to Kotor, though, so not mandatory), Kotor, Herceg-Novi, quick stop in Perast, Lovcen National Park (transport permitting) and possibly Lake Skadar. I found the capital, Podgorica, distinctly underwhelming, but it was a convenient base for side-trips to some of the above destinations. It may well be an unavoidable stop for reasons transportation.

Suggested mainland destinations in Croatia: Dubrovnik, Split, Zadar (somewhat reminiscent of Dubrovnik, so skippable), Istrian Peninsula, Plitvice Lakes National Park (top recommendation; stay near park the night before visit to beat the hordes of day-trippers onto the walkways), Zagreb.

Spain: I highly recommend Cuenca, which is an affordable and quick train trip from Valencia if you buy the AVE ticket early; otherwise, it will be painfully expensive. If you have access to a car through your friend, I also recommend the charming and unvisited-by-Americans Teruel. There's also bus service that seems to take less than 2 hours. An overnight in Teruel would be pleasant. Well, except for the heat; brace yourself for that. There's fast train service to both Barcelona and Madrid from Valencia. Which direction you go depends on your personal interests.

In response to the OP's second post:

I spent 3 months in Spain last summer, not getting to either Andalucía or Valencia, so there is certainly enough to keep you busy in northern Spain and Portugal for two months! I stand by my previous recommendations for Cuenca and Teruel.

With that amount of time the difficulty of moving between Spain and Portugal will not be a major issue. Just be aware that the border-crossing is something to be checked out ahead of time. Try to get a multi-city (open-jaw) airline ticket so you don't have to double-back to your point of origin on this trip. It will probably save you either a day's travel time or the cost of a short-hop flight.

Take weather into consideration in plotting your route. All other things being equal, I'd want to hit the northern coast of Spain earlier in the trip. Except for the far western bit around A Coruna, temperatures along the coast tend to be much cooler (and wetter) than the rest of Spain. I think early September would be a nicer time to be there than late October. I wonder about the possibility of flying into the Basque Country, traveling west to Galicia, south through Portugal, then back to Spain. You can cross the border from Lisbon to Madrid (I think on an overnight train; there must be some sort of bus service, too) and from the Algarve to Seville (that's by bus).

Thanks so much acraven and geovagriffith! Very helpful!