Advice on Poland and the Baltics

I'm planning a circular trip to Finland this july / august. I'm planning to ride out via Germany, Poland and the three Baltic States, taking the ferry from Tallinn to Helsinki. As I've never been to Poland, Latvia, Lithuania and Estonia, any advice of places to see, accomodation and any other useful advice would be appreciated. I will probably have six or seven days for this leg. Also, is it necessary to book the Tallin - Helsinki ferry in advance?

I may also decide to visit the TT-Nord shop in Hamburg, so just wondering how easily accessible is it for someone not too keen on riding in big cities?

TIA

Do a search for "Poland" as this topic has been aired several times recently