I'll arrive in Tallinn on May 25 on the ferry from Stockholm. I'm really excited about taking this ferry, because it's the same ferry route commemorated in Canticum Calamitatis Maritimae by Jaako Mäntyjärvi, in which I sang the narration solo with The Esoterics about 11 years ago. (Different actual ferry, of course, from the one that sank in 1994.)
[As an aside, Jaako Mäntyjärvi was born in Turku, Finland, which is one of my planned daytrips from Helsinki.]
The ferry departs from Stockholm at 5:45 p.m. and arrives in Tallinn at 10:00 a.m. It is supposedly an extraordinarily scenic cruise through the archipelago. And it's a fully featured cruise ship with great food and entertainment.
I'll have most of the day in Tallinn on Friday, and on Saturday I can hang out as long as I like before I cross by a much shorter ferry trip to Helsinki. There are six ferries, five of which are high-speed trips that take two hours.
I'm not sure how I'll spend my time in Tallinn, but I don't think it will be a problem, as the pictures I've seen lead me to expect just to enjoy wandering aimlessly.