I’m finishing a 7 in 1-month interrail pass currently (where I get 7 travel days to use within 30 days).
I travelled to 10 different countries; Holland, Belgium, Austria, Slovakia, Hungary, Poland, Czech Republic, Sweden, Denmark & Germany.
It really depends what you want to do.
I loved visiting Czech & Poland. While Slovakia & Hungary weren’t my cup of tea. I’m sure that Slovenia and the west would be nice too!
Holland is amazing - and definitely worth a visit while I personally feel like Belgium & Austria are overrated.
Sweden, Denmark & Germany are all expensive countries - especially Denmark! You can easily go over-budget from just a day of travel.
Night trains are what saved me lots on hotels and other bills, I definitely recommend some night trains.
Reservations were basically non-existent.
Only long-distance high-speed trains in Sweden, a few in Poland. Night trains too. Commuter trains (which are more common / better / faster!) are all unreserved seating.
ICE tends to have a lot of free seating available, without needing a reservation.
I did Copenhagen - Hamburg & I’d definitely book a seat.
Switzerland, Italy and those areas will be lovely too to visit - and close if you decide to start from Belgium / Netherlands.
Sometimes it’s cheaper to fly than get EuroStar reservations (it’s not cheap!), so keep that in mind too.
You can use your outbound and return “own country” journeys whenever - and they don’t need to be in order either (you can use return at the start of the month and outbound at the end).
You’re very flexible as you don’t need to worry about days running out, so you can hop from country to country if you don’t like anything.