You are of course entirely free to do anything you wish before the start of the tour in Ljubljana and after the end of the tour in Helsinki (and you can even join the tour a day or two late or leave it a day or two early if that fits in better with your plans, too).
To give you some ideas, here are some thoughts about what you might wish to do before or after the main tour.
If you are keen to add as many extra countries to your list of countries visited, there is an obvious ‘difficult to visit’ country nearby that cries out for you to go see – Liechtenstein. We’d do that by probably flying in to somewhere in Switzerland, then taking a train to the closest point in Switzerland to Liechtenstein, which would probably be Buchs. Simply walk across the bridge and you’re there.
After your visit to Liechtenstein, you might want to take a train on to Venice or Trieste, before then a train up to Ljubljana to meet with the start of the tour. This would get you three more countries (Switzerland, Liechtenstein and Italy), and could be done using a Eurail three country select pass (ie Switzerland, Italy, Slovenia).
Post Tour Suggestions
From Helsinki, you have two possible choices – well, of course you have many more than two, but the two we’d consider would either be going east to St Petersburg, or west through the rest of Scandinavia.
You can take a comfortable train to St Petersburg, where you’ll arrive at the peak of their White Night Festival – the time of year when the sun barely sets at night. This is a magical and romantic time in a city that is magical and romantic even on its worst days in winter. Note that you need a visa for travel to Russia.
The ‘white night’ sun almost never setting experience applies similarly in Tallinn, Helsinki, Stockholm and Oslo too during the period of time close to the longest day (21 June). But St Petersburg has made this into a city wide arts festival celebration, and is definitely the best place to experience and enjoy it.
Alternatively, you could use a Eurail Scandinavia Pass to give you train travel through Finland, Sweden, Norway and Denmark, in which case we’d travel to Turku in Finland, then take a ferry to Stockholm, a train to Oslo, then a train to Copenhagen. stopping overnight in at least Stockholm and Oslo, and maybe other places too – perhaps add a side trip up to Bergen, also.