Europe is large and diverse. My advice would be to pick a direction: from north to south, from east to west or something like that. Then just go on and see what happens. Sometimes you might want to stay a couple of days in a place, sometimes just drive on. Why don't you for example start in Madrid and see if you reach the Black Sea? Or start in Riga or Istanbul and see if you reach Portugal? If you are six people you can have two cars, and choose different detours if you like. That is good.
Scandinavia has much more wilderness than south of the Baltic Sea. Deep forests, dramatic mountain range and so on. Continental Europe have more impressive cities. (My favourite building actually lies just a few hundred meters from Europe's border to Asia - Hagia Sofia in Istanbul. It was built in the 6th century as a Christian cathedral, and converted to a mosque in the 15th century.)
Read up as much as you can. It will make your journey much more interesting.
You are not from here (Europe) I guess, so do check if you need visas. Easy enough for Americans, if that is your citizenship. Some African and Asian countries have less access, if that is your nationality.
Money - just bring a visa-card or similar. It will work everywhere. Many countries but not all use Euros.
It can be really hot in southern Europe in the middle of the summer, 35-40 C. Scandinavia - where I am from - is more pleasant, but it can be rainy here for a week or so.
Is English your main language? Then you will be able to communicate everywhere from west to the borders of the former soviet union. People speak good English in the Baltics as well, I am sure.
The rules on alcohol and driving are generally very strict. Just saying, cause I don't know where you are from.
Just don't rush on to the next place, that spoils all the fun. Hang around, talk to the locals. They will show you the best places.