re: Prague/Budapest/Vienna

You can definitely buy an EU adapter on Amazon for cheap. People don't use the same terminology, but generally an adapter is a cheap item that simply lets you plug your stuff in. You can get a multi-country adapter that'll often have 2-4 USB ports as well for more money (20ish bucks) but a plain EU adapter is just a few dollars.

A converter generally means power conversion to allow you to use certain devices by altering the voltage. In my experience this really only comes up with hairdryers and straighteners or more pro electronics (not standard laptops, cameras, etc.) and my wife has always had a dual voltage straightener that works fine without one. Almost any hotel in these cities will have a hair dryer.

In short, you very likely don't need a converter, and I certainly didn't for tons of electronics and whatnot in all three of these cities.