Sure. it takes a travelday but isnt a problem at all

You could even do a 4 country trip-easy to do in 1 day: Rotjeknor (that is how we all call that port-city)-Antwerpen-change-LIlle in FR-Brussel in BE-ICE (if it runs) to Kóln in DE-and then ICE back, change Utrecht.

@runner.on.rails  So for example if i want to go from roterdam to bruges and return the same day,i consider  2 days at rotterdam at the trip planning,is that right?

Thank you in advance

You can take as many trains as you'd like during any 24 hour travel day period. A travel day lasts from midnight. There are no restrictions as to how many borders you cross during that one travel day. The only possible restriction is that Interrailers can only travel in their country of residence on the out/inbound days. If you want to travel from Rotterdam to Bruges and return on the same day, then that will only be one travel day.

In order to make it clearer to me (its the first time i travel with interail pass) i only have to specify days and destinations and i can go wherevery i want between this days and destinations.

Thanks in advance

Yes you have to note the trains you use on the paper pass or in the Railplanner App. 

Please note that some trains need a reservation. Like Thalys high speed trains.