Baltic State Escape

I would say get your Russian visas asap as you never know how long it will take. Ours only took a week but others have taken several weeks.

You might look at St Peter Line (cruise) which would allow you to visit for up to 72 hours without a visa.

We took the St. Peter Line ferry (www.stpeterline.com) from Helsinki to St. Petersburg and return last June. You can visit St. Pete visa-free for 72 hours or less. In other words, three full days and two nights in St. Pete bracketed by a night on the ferry out and back. Then a short ferry hop to Tallinn. We've taken the bus (http://www.luxexpress.eu) from Tallinn to Riga, and then on to Lithuania. I'd miss Helsinki before skipping Riga or Vilnius. Why go back to Helsinki for the return flight when you can fly open jaw and return from Riga (or Tallinn or Vilnius)? You don't mention how much time you are allowing for the trip so it's kind of hard to answer the question about biting off more than you can chew! :) Have a great trip!

I agree that since getting a Russian visa is both expensive and a big hassle, if you can go visa-free to St. Petersburg by taking the cruise, do it. Helsinki did not delight me, but even if I did love it, it's relatively small; you can see the "designated attractions" in 1 or 2 days. I haven't been to Riga or Tallinn, so I can't comment on those. Do look into open-jaw flights (such as into Helsinki and out of Riga), and compare this with the extra cost of returning to Helsinki if you get a round trip flight instead. To find open jaw flights, don't look for 2 one-ways; instead, use the "multi city" option on Kayak, Orbitz, etc.

I think ferry is a good choice for getting around. Historically, the Baltic Sea was the front gate of all the Baltic capitals. If you skip anything, Riga would probably be the one. It's the furthest out of the way, so more time lost to travel, and a lot less developed for tourism (which may be a good thing). Stockholm is closer, so a natural stop to include with your others, and includes a great cruise through the archipelago (if you're on a ferry).

Russian Visa was a nightmare to obtain and very expensive. If you can avoid it then by all means do.