S2 has moved overseas and we are hoping to visit late this summer. Have gotten excellent advice from y’all for other trips, so figured I’d ask here if anyone has recommendations or other words of wisdom! Thanks!
About 5 years ago we flew into Riga, later took the Lux Express bus to Tallin, followed by the ferry to Helsinki, and train to the Turku archipelago area and Moomin World which was adorable. Finland was significantly more expensive than the Baltics. We enjoyed Finland and wouldn’t mind going again but we both really want to return to Latvia and Estonia.
In Lativa we loved the variety of architecture. A small museum of their occupations by the Soviets and Germans was very informative. It is easy to get to http://okupacijasmuzejs.lv/en/
The walled part of Tallin is every princess’s dream. My husband got his customary souvenir haircut from a neighborhood hairdresser and came out looking like a Russian oligarch. Awesome!
In Helsinki we took the ferry out to an island that used have important defenses.
https://en.wikipedia.org/wiki/Suomenlinna There was a prison on it - cute cottages surrounded by a low, maybe 3 feet, fence. The Hilton we stayed at was a tram ride out of town on the waterfront. We liked being in a residential neighborhood, picking groceries from a local shop, coffee in a local spot, walking the side streets etc. Manchester City soccer team was staying at the same time and it was fun to see the fans.
https://luxexpress.eu/en Comfortable coach buses
https://en.wikipedia.org/wiki/Moomin_World What a child’s theme park should be. The people/child watching was outstanding. The village it is in was very attractive.
I have to get to a meeting. I will think some more later.
I can’t help you with Ukraine or the Baltics but I’ve been to Finland twice. Some general observations: In Helsinki pretty much everyone under the age of 40 speaks perfect English. But in small towns out in the country that are off the beaten tourist path you can find yourself in situations where Finnish is the only language anyone speaks.
Helsinki is a sort of a standard European city. The best time to visit IMO is during the spring equinox or Mid-Summer’s Day as the Finns call it. It’s one big celebration that lasts late into the night (Why not? - the sun is still up that late). In warm weather there are lots of great local outdoor produce markets selling a lot of things that are rare in the US such as lingonberries and currants.
The Finns are typically a quiet and polite people. With respect to personal hygiene they are the cleanest people of on earth. They do have an alcoholism problem, and so it’s not unusual in some parts of town to see drunken men staggering about or passed out on the sidewalk, even at 9 or 10 o’clock in the morning. With increasing Americanization I don’t know whether this is still true, but when I was there I noticed that Finns never eat any food with their hands. Even such things as fast food hamburgers and fries were eaten with knife and fork. The Americans chowing down using their hands looked like barbarians in comparison.
The traditional Finnish cuisine is kind of bland - a lot of cold cuts, cheese, sausages, and pickled fish - even for breakfast - served with various types of breads and crackers. They are particularly big on potato dishes, and there a lots of varieties of potatoes there that you seldom or never see in the US. One sore point is that Finns claim to have invented the dish Swedish Meatballs and are offended that Swedes have somehow stolen the credit. They do have ethnic food restaurants in Helsinki, but I do not recommend them for spicy cuisines such as Mexican or Italian. They cut the spiciness way down to accommodate the bland tastes the Finns are used to. You can get things like reindeer venison in Finnish cuisine restaurants and in one place I had bear meat (didn’t like it - too dark and “heavy”). You can also get strange drinks such as a liqueur made from birch tree sap.
The Finnish language is not part of the Indo-European family of languages, meaning that it is completely unrelated to English and is thus incomprehensible. But road signs are usually in both Finnish and Swedish, and the Swedish often close enough to English to figure it out. For example a sign for “Airport” in Finnish is Lentokenttä, but in Swedish it is Flygplats.
I’m sure you will enjoy your trip.
I know this is in the opppsite direction of your travels, but you can, add Moldova. US citizenship easily allows to get a visitor visa. The largest underground winery/wine cellar in the world will blow your socks off. 
S2 has to go to Moldova soon to get his residence visa (he entered UA on a travel visa and now his employer will sponsor him for residence), so he is going to scout it out for us. Will tell him about the winery, as he’ll be there for 2-3 days while paperwork gets processed. SNowball, we like the local groceries for food and try to limit dining out to once a day. Give us bread, cheese, and sausage and we’re good!
We found some day tours that go point-to-point from Vilnius to Riga, and then Riga to Tallinn. Only eight people on the tour, and it would be kind of a cool way to get from A to B and see something beyond the cities. Will probably spend a day in each city in addition to the tour. We are mulling an overnight train, which would take us through Belarus. Train travel is somewhat problematic, as the USSR was never particularly interested in adding infrastructure in the Baltics. That will require a visa. Baltics are Schengen countries, so no visas needed.
Tallinn to Helsinki is a 2.5 hour ferry ride, and actually probably the best way to get home via Star Alliance FF miles, assuming we don’t head to Munich (my niece and her DH are stationed near there).
In German, airport is “Flugplatz,” so I can probably muddle through with Swedish signs based on cognates.
Correction: Due to brain failure, above I referred to Mid-Summer’s Day in Finland as the spring equinox. It is of course on the summer solstice - the longest day of the year.
I also spent a few days in Moldova in 2015. The main thing that struck me was the favorable exchange rate due to the Moldovan currency being very weak. The long taxi ride from the Chisinau airport to the center of town worked to some something like 5 or 6 dollars. A dinner for four at one of the nicest restaurants in town was in the $20 - $25 range. And the fare to ride the city tram came out to about 6 cents.
If you take the train from Romania or other non-Soviet countries into Moldova you have to stop at the border for a couple of hours while they jack up the train cars and remove the wheels and replace them with different gauge wheels to fit the old Soviet era tracks.
S will be taking a bus from Kiev to Chisinau, as the train goes through Transnistria, and that’s a place he’d prefer not to be! Folks he’s working with have been giving him suggestions for Chisinau, and then he hopes to go to Odessa for a couple days once he gets the re-entry visa.
We had similar experiences with Krakow in the past five years ($2 Uber rides, $20 dinner for two including wine). The hrivnia in UA is weak these days, too, and he has been able to live nicely on not much $$. He bought a Kiev metro card for $10 on his first day there and three weeks later, he said he had 19 rides left. Here, $10 on a Metro card wouldn’t even get him downtown and back!
In Chisinau we stayed in the Vispas Hotel, which was a nice and interesting place just around the corner from the Russian Embassy. It had spacious suites with hardwood floors. It was very quiet - apparently due to not having many guests. The front desk woman and bellman were often just sitting around waiting for someone to walk in. We were told by locals that some legitimate businesses were really fronts for criminal enterprises of one sort or another (money laundering, smuggling, etc.) run by wealthy oligarchs. Which made us wonder whether our hotel was one of those because it didn’t seem to be doing much business as a hotel, even though it was in fact a real and functioning hotel.
I spent a week in Helsinki and also visited Turku several years ago. My kid spent a summer with a family near Turku in high school and a college semester at UHelsinki.
Every day we were available to go to the island (Suomenlinna) the weather was bad (August visit). 
One thing I liked in Helsinki was the Temppeliaukio Church, which is built into rock. We also spent a pleasant day at Seurasaari Park, a short bus ride from Helsinki.
I will say, we loved Stockholm— you can take an overnight boat from Helsinki, spend a day in Stockholm, and take a night boat back to Helsinki, We spent a few days in Stockholm and Visby (island south of Stockholm), but even a day there would be fun. And the boat trip through the archipelago outside Stockholm at dawn or dusk is lovely.
Lol, my kid had some interesting experiences in that T… place, CD. They keep an eye on all Americans, it seems. The winery in Chisinau is called Cricova. We went there on a private guided tour, but there are guided tours in English offered at the winery.