In my early 20's (a time before cell phones, internet, portable GPS devices), my friends and I used to book our tickets to either Paris or Amsterdam and book the first night at a hotel plus our rental car for our 2 or 3 week trip. It started as a thing we had to do after university and before starting our careers and became an annual event to get away from our jobs. We took a good road map, money, and a large suitcase full of clothes each. Our starting point was usually Paris because of the rental car prices that were better back then from that hub. We looked at Eurail passes but none of us were too keen on taking trains and lugging our belongings around. We were young and liked the idea of driving in another country (and still do today). There were better priced hotels away from city centers and parking was much easier. Some hotels chains (such as Scandic Hotels) had a pass system - pre-paid vouchers for one or more night stays at any of their hotels and you can show up to the hotel and if they had a room, you could stay there. Otherwise we would look for vacancy signs at the hotels when we passed by (remember those?). My parents did the same thing when we went on our summer vacation road trips.
We would do road trips that would cover France, Belgium, Holland, Germany, Denmark, Norway, Sweden, Finland, Switzerland, Italy. Other than our flights and first night, and rental car bookings, nothing was planned. We would go out at night to clubs and found ourselves at strip clubs with some locals we met. Who plans for that? Most people from Northern Europe headed south during the summer months, so finding hotels was not really a problem.
We met lots of interesting people and ironically, the three of us that traveled together ended up marrying women from Europe (Italian, Dutch, and Swiss) and are all still married today. One even moved to Italy after his wife had a hard time adjusting to life here.
It's much easier to do what we did then now with smart phones guiding your trips, finding vacant rooms for you, and good places to eat.