Possible ways to get from St. Petersburg to Helsinki (or the other way around) [closed]

I'm looking for ways to get there without a private car and a group of 4-6 people in the middle of May. I'll name some of the possible solutions I've found, but I wasn't able to get much information on those. I hope that some of you are locals or did already travel this distance to help me to evaluate the best possible way.

1. Plane: Boring and extremely expensive for a flight < 1h (~130€).

2. Train: The fastest one is pretty expensive too and I didn't find any reports (if it is worthy) nor a timetable for May '18.

The slower one is probably pretty cool as you are able to see more of the landscape but I didn't find any information for this one.

3. Ferry: This one is the probably the most interesting one but I wasn't able to find any information about pricing and duration. There even wasn't a timetable for any timespan.

4.Bus: Probably the worst one as it takes really long and is most likely a bad experience, even though it might be the cheapest one.

5.Car rental: I didn't find any company that has an offer for those two locations (I did only a little research on German websites).

I'm not necessarily looking for the cheapest and fastest method, but also for a way to travel the distance and having a great time with my friends and to see something from the two countries. As we don't care which of the two cities we visit first, both directions can be taken into account.