Winter Adult family trip-9 days start and finish at Helsinki-open to suggestions!

Hi we are traveling in mid Dec 2015 as a family with two young adults on uni break (20+ 22) and land in Helsinki on 12 Dec . We have 9 days before our flight to the UK and definitely wish incorporate a trip to Tallinn for a couple of nights . For the rest we were considering going back to Saariselka, northern Lapland for 5 nights.as we loved the winter landscapes and skiing..However we have been there before and are also considering other options of things to see, while we are in northern Europe.
I have had a look at a few sites and see that there are cruises /ferries which go between Riga, Stockholm, Helsinki and Tallinn and wondered if anyone had experienced that in winter and how suitable a cruise might be for 2 young adults.??We have never been on a cruise but it could be an option, depending on costs and itineraries.
We will have our warm clothing with us and enjoy old towns and buildings, scenery, Christmas markets, market food, cafes, bars, music if any concerts might be on, exploring, walking etc. (Prefer to book an apartment if 2 nights + but ok with hotels for one night stops). Budget mid range.self catering a bit helps keep our costs down but we tend to do that when we get a place for 5-7 days. Whilst keen to see places, too many stops can be tiresome so any advice appreciated. We have been to St Petersberg previously so will not include that this time, brilliant as it was.!