With the research that Audrey and I conducted, we decided that we should go and see the UNESCO listed Old Town of Riga while we were so close.  To do that it seemed that the best way to get there was to go on a ferry from Helsinki to Talllinn and then by bus down to Riga.  I'm glad we went there.  Tallinn is another oldish City in the former Swedish/German/Russian empires that also has a remarkable Old City complete with Walls and Towers and Gates and hidden underground passages and an upper section and a lower section.  The streets are cobblestoned and narrow, the buildings are not tall but tall enough to feel like they are crowding you when you walk between them and the shops and resturants keep in turn with the feel by looking ancient.  And we both felt that the people were really friendly, really genuine towards us.  We liked it that much that we ended up staying a couple of extra days just to see a bit more of Tallinn.
Once again we bought the Tourist Card for Tallinn, surprisingly called the Tallinn Card, and spent our time visiting the Sites and Museums that were associated with it.  The first place we went to was the Open Air Museum, a place just outside the city proper, in amoungst the trees and spreadout.  All the exhibits are genuine buildings that were found around Estonia and that can be dated accurately, for instance, the wooden church (only 2 left in Estonia) was built in the early 19th Century based on the '1813' engraving on the front door.  In total there are 68 farmhouses arranged into North, South and West styles.  This was my first visit to a site like this and I found it very enjoyable.  The Tallinn Card allowed us to enter pretty much all major tourist areas, like the Tallinn TV Tower (where we watched a very funny short film about the Tower), the Seaplane Harbour (was once the world's largest concrete open space) and the St Olaf's Church (was once the largest building in the world, from 1549 to 1625).  The other thing that I would suggest that travellers do is the 'Hop-On-Hop-Off' buses that a lot of Cities have these days as they give a good, but quick, overview of the city that you are visiting.  This way you can have a look at the City first and then decide what you want to look at closer.  We have found it a good way of going about things.  Tallinn overall was a very good place to visit
Riga on the hand was a little disappointing.  Having just come from Tallinn, and reading about the 'Old Town' I was expecting something in the 'wow' areas but, to me, it just wasn't up to scratch.  Maybe if we hadn't already seen what we have seen it St Petersburg or Tallinn or even Helsinki it may have been better, maybe if I hadn't read all the great stuff about it, maybe if I'd never heard about Riga, it would have been good.  Maybe my standards are just becoming too high after the travel that Audrey and I have done so far.
Having said that they are spots that deserve a mention; the canal that circles the 'Old Town' is very pretty and St Peter's Church looks outstanding considering it was destroyed during WWII.  And in truth there are a lot of history to explore in the Museum's on any other days apart from Monday (guess what day we chose to walk the 'Old City').