Baltic trip

We have been to all those places except Helsinki and they are great.

Consider Gate 1 Travel's tour that includes those places:
https://www.gate1travel.com/tour/central-europe-tours-12dclbaltich26.aspx

Also, Gate 1 has more tours of the Baltics. We have done several tours with Gate 1 and they have been excellent with low prices.

I went to Vilnius, Riga and Tallinn on a Road Scholar tour a few years ago and it is a fascinating region with so much history and things to see. The thing I really liked about Road Scholar was that we had a lecture in each city from a local expert on topics such as current events, recent history, social topics, etc. Definitely go there!
I also noticed that Overseas Adventure Travel has a small group tour that goes to all those cities except Gdansk, and I've heard good things about their tours.

Last summer we were on a road trip through the baltics, so we did Vilnius, Riga, Tallinn and some in between - super nice trip. Helsinki would have been just a small jump, but we had no time anymore. Easily doable by rental, almost all people spoke English - well except for some old ladies selling fruit at a farmers market, but that worked by pointing fingers.

Here is a link to my trip report from September 2024.
https://community.ricksteves.com/travel-forum/trip-reports/baltics-september-2024

I had already been to Gdansk as part of a trip through Poland and haven’t yet spent time in Helsinki.

Instead of those cities, I included time in secondary stops in between Vilnius, Riga, and Tallinn.

I have only been to Helsinki and Tallin (last summer) and I would suggest that you start your trip with Helsinki as it was super underwhelming after visiting Tallin. Estonia is beautiful!

Are you planning to fly from Gdansk to Vilnius? You may want to check whether there are non-stop or connecting flights that you are happy with.

When I considered this group of cities in 2023, I wasn't happy with my flight options from Gdansk, which connected back through Warsaw (LOT Airlines) or Copenhagen (SAS Airlines.) Since I wanted to visit Warsaw anyway, I started there, then flew (non-stop) to Vilnius, saving Gdansk for a 2024 trip.

My suggestions and comments on hotels, transfers and sights are in my 2023 trip report:
https://community.ricksteves.com/travel-forum/trip-reports/3-weeks-4-capitals-solo-after-warsaw-in-3-baltic-capitals

I visited all in either 2018 (Gdansk) or 2022 (the others) and liked them all. I'd agree that (unless you're nuts for Art Nouveau architecture) Helsinki may have less immediate appeal than the others, though it has worthwhile art museums and some interesting modern architecture.

I spent roughly 1 week in each of the listed cities. That's what I needed to cover my hit list, which was focused on art museums, 20th-century history museums and architecture. And those are not the only interesting cities in their respective countries.

It's true that travel between Lithuania and Gdansk isn't necessarily straightforward. Russia (in the form of Kaliningrad) gets in the way.

A couple of economic points:

• Helsinki is quite a bit more expensive than the other cities, and I suspect Gdansk is somewhat more expensive than the other three.

• I know overnight-ferry fares vary, with late ticket purchase sometimes being pricey. I don't know whether the same applies to the daytime link between Helsinki and Tallinn.

How much time are you allocating to this trip? And do you think you'll want to explore
the countryside, or at least get away from the cities?

I ask because you could easily spend a week+ in each of Estonia, Latvia, and Lithuania
and barely scratch the surface.

I have nothing against Gdansk, but it seems a bit out of the way and I might consider
leaving it out depending on how much time you have.

Also, while there are trains in the Baltic countries, buses are a very reasonable way to
get between the cities.

There's no need to use a tour if you don't fancy it as all four cities can easily be explored on your own. English is widely spoken and there are a number of excellent guide books available. I did opt for a personal guide at the KGB building in Riga and also the Museum of the Occupation of Latvia, both were relatively inexpensive and provided a more detailed insight to the history of Latvia.

For my most recent Baltic trip I left out Vilnius because there was no straightforward way to incorporate it with Gdansk and Riga. I flew into Riga from London, flew Riga to Gdansk, train from Gdansk to Warsaw and flew Warsaw to London (I didn't want to fly from Gdansk to Luton or Stansted as they were the only options so I chose to revisit Warsaw for a day and fly back to Heathrow).

Whilst in both cities I walked most of the time as they're quite compact and easily walkable and for the occasions where walking was unfeasible I used Bolt, cheap and efficient and very easy to use with the app. For my visit to Malbork Castle I took the train from Gdansk train station. Very easy to do and despite knowing almost no Polish I had no issues with buying tickets and getting on the right train.