Trains to/from France and long distance trains in Sweden needs compulsory reservations. In Denmark*, Germany, Netherlands** or Belgium** you don’t need a reservation but you can buy one for long distance trains. 

You can start to plan with timetable websites like German railways bahn.com. Please also think about to do some stops in between, because from Stockholm to Paris is a long way. 

*In summer trains Germany-Denmark needs a reservation

**Except Thalys and Eurostar highspeed trains, compulsory reservation. 

Or else visit seat61.com-it overviews all overland routings to/fro UK to all EUR countries and tells how to book etc. Thats mostly for normal tickets. Just combine the routings for your plans.

Probably best way to organise it-without too many undue stops: Snalltaget overnight to Hamburg-MUST REServe/book. IN DE for these shiny white ICE you can, but do not need to book-easily done via bahn.de (which also has a train planner that works for nearly all of EUR), very much advisable for peak times.

IF you want to save money-the FRench superfast TGV trains OVER the border (also Thalys) cost quite a lot extra to pay-you can avoid this by hopping over border with local trains. Go to FRAnkfurt-choose-via Karsruhe-Offenburg-Strasbourg or Saarbrucken, or if you fancy it Luxembourg (for a short stop), then into FR the local TGV to ´la ville lumiëre´. Can do-if taking these expensive trains- in around 24 hrs. and using 2 passdays.

The quickest way south from Stockholm is to take the night train to Berlin. A seat is 199 SEK with an Interrail pass and a bed in a shared compartment is 399-499 SEK. Please note that there is construction work going on both in Sweden and Denmark during the summer so the train doesn't run every day.

https://www.snalltaget.se/en/berlin