Where/How do you get ideas for places to travel?

Me and my friends are planning a big trip to Europe. Our first idea is Italy, Switzerland, Spain, France and another less visited country(something like Croatia, Montenegro, Slovenia, ALbania, etc).

The problem is we are still trying to figure it out the best places. We already know some cities and attractions that are certain, but we are scared to miss other cool places with pretty views because it isn't ''famous enough''. Where can i find actual good ideas of places, and not the basics like museums, cathedrals and stuff like that?