This app was mentioned in 6 comments, with an average of 2.17 upvotes
If travelling Europe, I found that: Rail Planner https://play.google.com/store/apps/details?id=de.hafas.android.eurail&hl=nl is really useful. An offline rail route planner.
It was built by and for use with Interrail/Eurail, but of course works without. It's purely a route planner so you'll have to deal with the tickets yourself.
I'd recommend using the interail rail planner app, here is what it says about ferries from Stockholm to Helsinki (via Mariehamn)
Tallink Silja Oy
Offer: Enjoy a 20-40% discount on online bookings for Tallinnk Silja Oy ferries operating the following routes to and from Sweden:
Helsinki-Stockholm via Mariehamn
...
Info: To make a booking please go to www.tallinksilja.com. To make contact by telephone, please use one of the following numbers: Finland +358 (0)600 15700 or Sweden +46 (0)822 2140. Offer code: EURAIL
Please note: The discount varies depending on departure date and booking situation. The discount does not apply to port taxes.
Link for the app: https://play.google.com/store/apps/details?id=de.hafas.android.eurail
I suggest you to use a handy app: Rail Planner. You input starting and end point to get your path. It's 100% offline, so you can use it even when you're here in Europe without being robbed by telcos. Also, it has options to avoid trains with compulsory reservation so, if you're in a hurry and haven't booked in advance, you can get on a regional train with your eurail ticket and nothing else.
It would be smart to use night trains but those often needs reservation, especially long routes. You might also be forced to pay for bunk beds since some night trains simply do not have seats (if I'm not mistaken this info is available through that app).
Bunk beds are often WAY more expensive than a cheap hostel. For example I almost had to pay 70€ to go from Munich to Verona. Being a cheap bastard I travelled through central Europe for 30 hours straight with a total expense of 5€, instead. Indeed it was quite an adventure ;)
By the way, going from Gothenburg in Sweden to Paris in 7 days means you won't get off the train, IMHO.
Not affiliated but I like the rail planner app (https://play.google.com/store/apps/details?id=de.hafas.android.eurail&hl=en) which works fully offline and has an option in the route planner for trains without a compulsory reservation. I wouldn't risk it at the station if you already know the trains to be full. If you don't want/arnt able to download it post your routes and I'll get back to you.
The app from interrail themselves is also very useful. https://play.google.com/store/apps/details?id=de.hafas.android.eurail