You could plan a day trip to the old town in Tallin Estonia. There is a fast ferry from Helsinki to Tallin. We went in the morning and returned late afternoon. Was a fun day.

We took the Silja Line overnight ferry from Helsinki to Stockholm. The dinner buffet was wonderful.
There is a ferry from Turku to Stockholm also.

Perhaps you could go from Stockholm to Aland Island and then to Turku, Tampere and end up in Helsinki. Then take the overnight ferry back to Stockholm.

Helsinki and Tallinn are both fine walking cities and worth a day (latter) or two (former). For more, see my phototravelogue at http://www.kodakgallery.com/I.jsp?c=f363d0s.ben2fb20&x=0&y=-95hdl3

definitely a day trip to tallinn

SemiMike - loved your photos of Helsinki and Tallinn. We were in Tallinn in 2000. They were busy doing a lot of renovation work that year. I could see from your photos that much has been done.

I love Helsinki and it was fun to look at your photos and remember the places we were.

Thanks for the suggestions, and to SemiMike, for sharing your photos.

I'd suggest you to make a round-trip Stockholm-Helsinki-Tampere-Turku-Åland-Stockholm. This can be easily done by ferry and train. First take an overnight ferry from Stockholm to Helsinki. Viking Line is cheaper, Silja Line has better food. Both are OK and they leave Stockholm at around 5 pm and arrive in Helsinki at around 9 am. 3 days in Helsinki is just fine, of which you could think of spending one day in Tallinn. Tampere and Turku are pretty small, and they both can be explored in one day. Journey between the cities takes approx. 2 hours by train (www.vr.fi). That would be 5 days altogether. The Thing in Finland is the nature, not the cities, I think. In September the weather still can be nice and warm, if it's sunny, so you should seriously think of experiencing the nature. Åland should be good for this (even though I always suggest the lakeland in Eastern Finland, but you won't be having time to go there). Viking Line and Silja Line take you from Turku to Mariehamn, Åland in couple of hours (take the morning ferry, otherwise you arrive in the middle of the night). From Mariehamn you can continue your trip back to Stockholm. Hope this helps you! Next time spend some more time (in summer) and go cruising from Savonlinna to Kuopio in Eastern Finland!

Thanks, CatRat - these are very helpful suggestions! I hope to have more time on the next trip!