In Finland, VR sells interrail passes. A pass for 22 days was on -15% sale and costed 419e. https://www.vr.fi/cs/vr/en/interrail-english
Trains and connections were really easy to check via the Rail Planner app
https://www.eurail.com/en/plan-your-trip/rail-planner-app
Also Rome2Rio app seems handy
https://www.rome2rio.com
Boat tickets were 86e from Turku to Stockholm, 16e from Gothenburg to Fredrikshavn and 180e from Travemünde to Helsinki with Finnlines, although with the interrail ticket there would have been a 50% discount.
Seat reservations were compulsory only in Sweden, but were good to have in Germany for always getting seats in the trains which apparently are pretty full in the summer. The reservation portal at https://www.interrail.eu/en/reservations was CRAP, but reservations were quite easy to make via websites of the train companies of the country where the train departs: SJ for Sweden (~8+e), DB for Germany (4,5e), ÖBB for Austria (3,5e), Trenitalia (~10e) for Italy etc.
Very detailed instructions were found here:
https://www.interrail.eu/en/reservations/how-do-i-book-my-reservations
Deutsche Bahn’s app is great!
https://www.bahn.com/en/view/booking-information/booking/db-navigator-app.shtml
Night trains seem to be bit special and should apparently always be booked well beforehand: https://www.interrail.eu/en/plan-your-trip/trains-europe/night-trains
We paid 44e for beds with the ÖBB Nightjet Padova-Munich which also included a continental breakfast The reservation needed to be done via phone with ÖBB at +4351717
Deutsche Bahn’s phone service was crappy and with a long waiting time at +491806996633
Also got very helpful tips in fb groups
”Maata pitkin matkustavat”
https://m.facebook.com/profile.php?id=867083580048037&ref=content_filter
”Interrail travellers official”
https://m.facebook.com/groups/436442960147237
Accomodation was easy to book with Booking.com and Airbnb apps, price range being between 25-70e/night per person.
Rent for the car from Europcar with Oona and Valtteri costed 290e for 10 days.
Most of the time I rented bikes from hotels or bike-sharing apps, for example the Donkey bikes (~10e/day) were available in all of the cities visited in Germany: https://www.donkey.bike
Timewise it takes roughly
26 h from Helsinki to get to Copenhagen (ship to Stockholm + train 7h) or
36 h to Berlin (ship to Travemünde + 4h train).
But then
Berlin-Munich is only 4h and
Munich-Venice 6h,
and when leaving and arriving to center of the cities takes almost the same as flying.