Sounds like a nice idea, I’m planning something similar this summer.
Do you rather like towns or are you more into nature?
For towns, I’d consider Colmar or Strasbourg, Metz and Luxembourg if you want to go that way. Then doing some places in Belgium like Brugge or Gent. On that route, you won’t need to care about reservations at all.
If you like nature, I recommend the Black Forest region which is rather well connected by rail.
Maybe think about a short detour into the Netherlands (Maastricht) or Germany (Cologne, Dusseldorf).
Thank you so much. That is really helpful. I think for this trip we would prefer to do towns, but we do love a bit of nature as well so we will have a look at all the places you’ve suggested. I love the idea of a short tour into the Netherlands because we hadn’t thought of that. Now time to get planning! Thank you 🤩
Since you’re starting in Interlaken I would opt to take the EC6 train which originates there and heads up to Dortmund (although I thought it went to Hamburg). I’ve nothing particular against Dortmund, but if it was me I would go as far as Cologne and then spend the next few days heading west towards Brussels.
If you have already “done” Switzerland then the EC6 route up the Rhine is a nice way to see Germany.
Since you’re starting in Interlaken I would opt to take the EC6 train which originates there and heads up to Dortmund (although I thought it went to Hamburg).
It is terminating in Dortmund for a while now and sadly will be discontinued by the next timetable change in December.
Thank you very much for the advice. I’ll take a look at that. 
