First time to Greece, without a car... and questions about onward travel to Bulgaria

And any tips for traveling outside of Greece into North Macedonia or
Bulgaria, by train (or bus, if need be)?

A good start is https://www.seat61.com/international-trains/trains-from-Athens.htm for the options. Athens-Thessaloniki is easy by train, relatively fast and modern trains connect the two cities. But going north of Thessaloniki is a bit more challenging.

I believe the trickiest/slowest places to get to will be Folegandros and Meteora, but let's see what our Greece experts say.

Crete is large; giving it adequate time could push you beyond the time you have available.

Santorini is really expensive.

The last time I looked there was no rail service from Greece into either Northern Macedonia or Bulgaria. Rome2Rio.com says there are three bus companies providing service between Thessaloniki and Sofia. If you keep drilling down on that website, you'll find the names of those companies and links to their websites. Don't trust the fares, travel times or frequencies shown on Rome2Rio.com! Even the information on the companies' websites should be verified on the ground. My policy for bus travel in the Balkans is not to believe any schedule (or the existence of a bus) until I see the information posted at the outbound bus station. Border-crossing buses run infrequently (Rome2Rio's info may be wrong about the frequency), so it is prudent to buy your ticket at least one day ahead of time if you are in a position to do so.

Bulgaria is very interesting. I especially liked Plovdiv and Veliko Trnovo. However, travel there is slow (no fast trains--figure about 30 mph.), so be sure you allow extra time for getting around. On the other hand Bulgaria is very inexpensive.

I cannot address Northern Macedonia, but you can start with Rome2Rio to explore your transportation options. A sense of adventure would be essential there. Among other things, I think you will be run into hole-in-the-ground toilets at some (maybe all) of the bus comfort stops. You'll find English spoken at hotels and restaurants catering to western tourists. Otherwise, your best bet is the young people. This applies to Bulgaria and (I assume) North Macedonia. Greece should generally be no issue. They've been polyglots for centuries.

You can check skyscanner.com to see whether by luck there might be non-stop flights between Athens and Sofia or Skopje.

Eliza: too many places, too little time. Traveling by bus to Nafplio, the day trips, Delphi, Meteora, etc consumes way too much time for each leg. The schedules only tell part of the story. The same applies to ferry travel to the islands. Renting a car would make a big difference as you can travel at your schedule and combine some places, not needing a day apiece as with a bus. For instance, you can stop at Nemea, Mycenae, Tiryns on the way to Nafplio and,Epidaurus, Corinth on the way back. The driving is easy. Central Greece is more mountainous. Crete is enough for 3 weeks by itself - lots of old stuff. Santorini is beautiful but a pricey tourist island (even a lot of the locals don't winter there). Naxos is a real Greek island. RS tours goes to Hydra because of the short travel time. I think it would be better to limit your travels to be able to really see and experience what you are traveling for.

Elizebeth, lucky you, with 3 weeks to get acquainted with Greece! We'll try to help you have a wonderful First Visit, if you'll do your part, by providing important info affecting our advice. We'd waste time, effort giving irrelevant tips without Key facts:
• WHEN? March VERY different from August!! Both in cost & what's accessible.
• EVER DRIVE? U say no car, but I'm guessing that's budgetary because in other threads u ask about rentals; would u be willing to rent for just a day in key spots, to places hard to reach via no or infrequent transit? Do u have Intl. Drivng Permit? (required for renting)
• ARRIVING/LEAVING - probably by plane, almost no train service to/from Greece

Just as an example, these thoughts occur to me, for a itinerary/sequence... (IF you want to go to N. Macedonia @ end)
• Arrive ATH Airport - switch to KTEL (intercity) bus to NAFPLIO, (great intro to ancient hist, arch, landscapes etc etc!) for 4 days, including car 1 day for "ruin-a-thon" in area (Mycenae-Nemea-Epidaurus). Travel day (1) =Bus to ATH, 5:30 ferry to NAXOS
• ISLANDS - NAXOS (4+ Days) SANTORINI (3? Days) - ?? FOlegandros?? or HERAKLION Crete?? O'nite ferry to Piraeus?
• MAINLAND FINALE - KTEL bus to DELPHI (1 night) - ATHENS (4 days) - Train to THESSALONIKI (3 days) - VERGINA (1 day)
-- as u can see, a LOT depends on what time of year (Cold? Hot? Rain? Flowers? Beaches? Sunsets? )

• YOUR INFO SOURCES - do u have/can u get a GOOD Guidebook like Rough Guide to Greece; internet helpful but explaining Crete on a travel forum/outlining a tour plan is HARD.
• CRETE - It's really a country in itself (Santorini is 12 miles long: Crete is 161, with 3 major mountain ranges). Here's an online map; click & it gets huuge, navigate w. side-slides https://www.explorecrete.com/crete-maps/images/Crete-eot.png I"ve visited Crete 5-6 times, 5+ days each time, WITH a car, and haven't covered East Region yet. Without a car, even partial tour could take weeks.

So -- as I said -- the more you share, the more we can help!