Ithaca, Kephalonia and Zakynthos in May

Are the Ionian Islands a deliberate or random choice?
(if I understood correctly your trip to Italy is to be dissociated from your trip plan to Greece)

Yes, the Italy trip is separate. To be honest, I just started googling info on Greek islands and saw several sites suggesting Ithaca for a more low key vacation. After reading about various islands, those three stood out as some I might like.

Ithaca is indeed an island off the beaten track of the usual Greek lovers islands.
Here is some practical information, hoping that it will be useful to you:

The easiest route would be to fly to Kefalonia first via Athens.
Aegean/Olympic fly regularly from Athens to Kefalonia
Also note that United and Aegean/Olympic has a codeshare agreement. You could have a Dulles-Kefalonia with just one ticket.

Since Kefalonia airport is international, there may be other possibilities by passing through another European city than Athens. (but not with Aegean).

Ferries from Kefalonia (from the port of Sami) to Ithaca are operated by Levante Ferries and Ionion Pelagos

https://www.levanteferries.com/en

http://ionionpelagos.com/en/

You can also book private boat transport with Kavallieratos.

https://www.ithacagreece.com/ithaca-transfers/ithacatransfers.html

On the other hand, to go to Zante from Kefalonia is a little more complicated, there is no direct ferry connection, you have to go through Kilini, on the mainland, again with Levante Ferries. That said I don't think Zakynthos would be interesting for you, and 2 weeks, including transport times, is quite short.

You will find some additional information (among others) here:

https://www.greeka.com/ionian/ithaca/

https://www.chasingthedonkey.com/greek-ionian-islands-greece/

The websites were helpful, thank you! One of my concerns is how to best get around once on the islands. I'm not sure I want to rent a car, although maybe that's best in some cases. I'll have to do more research on that. Also, one of things I'm trying to be cognizant of is being realistic about how much I can see in a limited amount of time. It's that tricky balance of planning too much vs not planning enough since I like to stay somewhat busy on vacations.

As much as possible, I would like to wander through towns and villages and get out into nature and see unique sites. I don't mind stairs or hills and I'd love to find some waterfalls or overlooks or other scenic views. While I enjoy walking or sitting on beaches to reflect for a bit, I'm not one to lay around on a beach all day. I guess my solo trips are more about seeing new things and less about relaxing. Oh, and I'm definitely not interested in night life or the party scene. I tend to explore all day, then settle in early at night, although wandering through a well lit town at night once in a while can be nice too.

I haven't traveled all that much, but I'm already realizing that narrowing down a plan once I start thinking about the next place to visit is not my strength, so I appreciate any guidance I can get!

You still have time to refine your project. As it seems that according to your other posts you are planning a trip to Italy, you will use this experience to avoid making some mistakes, especially in the balance between things to see and do in relation to the time you have available, and aslso especially if you prefer to settle down and hang around rather than running to catch a train or a plane.

A trip to the Greek islands has nothing to do with traveling between two places on the mainland.
With the ferries, it is necessary to take into account the itineraries and timetables which do not allow you to make all the trips you want when you want. And when possible it takes time, at least half a day even to travel a few nautical miles between two islands.

Once on an island you don't have to rent a car every day. There are bus networks.
However Kephalonia being a large island, moreover quite mountainous, a rental car is an option to consider seriously mainly because the buses are like the ferries, they do not go anywhere at any time.

Here is another website to keep in your bookmarks, this is the one for bus services in Kefalonia, you will find useful information there, including timetables and bus routes:

https://ktelkefalonias.gr/en/tourist-guide/meet-kefalonia

The same site offers practical information about Ithaca here:

https://ktelkefalonias.gr/en/ithaca-sector