I do remember the border crossings from West Germany to East Germany when I was still a child, the border crossings from France to Spain to Portugal with the need to change money at one border or another. This is all gone, making it easy to travel in a united Europe with the establishment of the European Union as well as the monetary union with the Euro.
In Europe most countries are quite small and one can visit a number of countries in just one day without any or very little border formalities.
The countries I visited in Europe on various and multiple occasions over a period of three decades are:
Denmark, Sweden, Norway, Finland, Netherlands, Luxemburg, Belgium, France, Spain, Portugal, Gibraltar, Italy, Croatia, Serbia, Bosnia Herzegovina, Macedonia, Russian Federation, Hungary, Montenegro, Lithuania, Latvia, Estonia, Slovenia, Czech Republic, Slovakia, Greece, Turkey, Romania, Bulgaria, Albania, Austria, Poland, Liechtenstein, Switzerland, Monaco, Ukraine, Isle of Man, Andorra and the United Kingdom.