Are you from UK?
This site has all the info you broadly ask for: seat61.com. It is aimed at describing all overland travel from UK to all EUR countries and actual state-will also give details on alternatives, its not just for those steel monsters on rails.
AND simply start with using this site itself: country info. This will also give basics for all countries in the scheme and indeed scenic routes, hidden surprises etc.
In general: as on all forums, such broad questions without nay hint or clue of what people interest will lead to nothing or just a very few very general remarks.
For very specific railfan/´idiots´; use a trainforum like railforum.co.uk-about any big country has similar in its own language.
General note: for many short outandback trips spread out over long time an IR might not be best solution. Most countries also offer various other much cheaper ride-at-will passes, for the day/week or whatever.
A good base in my eyes is Vienna or the cheaper Bratislava
From Vienna you have dailytrains to Budapest,Prague,Brno,Tatra Mountains, Warsaw,Krakow,Berlin,Dresden,Munich,Salzburg,Graz,Lubljana,Zürich,Bucharest (as Nighttrain),Venice,Rome/Milan(as Nighttrain)
Bratislava and Vienna are just 1hour by train from each other :)
Another good base would be Frankfurt in Germany (Strassbourg,Brussels,Amsterdam,Rhine & Mosel Valley,Black forrest, even a direct train do Budapest 
, Paris,Zürich,Chur,Interlaken, Bern)
There are so many scenic routes to mention or routes with special trains And it´s hard to answer it complete :D
I enjoy traveling by train to And start this evening a “daytrip to Switzerland” from Berlin
Overnight Berlin - Basel - then Interlaken - Lucerne - Ship - Mount Rigi - Zürich (Uetliberg) and finally catch a train overnight back to Berlin :D
The question is what you prefer (Special trains like Loco hauled or Units,
Strange ways of going For example the New RAiljet from Frankfurt via Stuttgart - Lindau/Bregenz - Feldkirch - Innsbruck - Salzburg to Vienna) Instead of the direct Route Frankfurt - Munich - Vienna or Frankfurt - Passau - Vienna :D
Scenic Routes?
The trains with the good dinning cars? (one of my passions )
Branch lines?
Thank you @mcadv I appreciate your advice and suggestions… I know what you mean, by “question” was pretty open-ended 
I suppose I really like to hear people’s stories, over and above the various resources and articles out there. Since writing this, though, I have been digging deeper into my research on the Intrerrail site and elsewhere, and my plans are coming together nicely. Thanks again for taking the time to write back.
Hey @seewulf, many thanks for your suggestions. Honestly, I love the look of all those routes you suggested… I’m a big dreamer who loves trains and adventures 
I’d love to spend some time in rural Germany and the Black Forest sometime, and I’ve been fortunate to visit cities in Switzerland and Vienna… so many beautiful options in Europe.
I’m curious to hear more about these “trains with the good dining cars” :)
ps. I’m keeping an eye out for the 17th edition of the Europe by Rail book, if you weren’t aware of it already
In my eyes in the wrong direction
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