Touring Europe was once done by rich folks whose servants struggled with enormous steamer trunks on every stop. Lucky for them the stops were infrequent; touring was once done the slow way out of necessity, as transportation was primitive.
Then came coach tours. The common folk got to join in on the fun--but they had to go on tours because there weren't many guidebooks. Now, you can take on European touring on your own. Finally. You can rent a car or take advantage of Europe's fine and extensive rail network.
Going It Alone or Guided Tours?
Spending a whole week or two on a bus tour of Europe is exhausting and only suitable for a select few people. On the other hand, it's very hard to fit everything into a trip when planning it all by yourself. Guided day trips are an especially useful for fitting in a city you could never get the most out of in such a short period of time when traveling alone. Where appropriate, I've suggested guided day trips worth taking on your tour.
On your tour of Northern Europe, start in London, then continue your tour to compelling destinations in France, Belgium, the Netherlands, and Germany. You can take this tour by train on the European rail system quite easily. You can also do it in a car, but finding parking in the larger cities like London and Amsterdam can be quite a deterrent.
The advantage of this itinerary is that the distance between destinations is quite short, so your days won't be spent entirely in transit.
Direct Destinations From London: You can take the train directly to Brussels, Paris, and Lille. Read more about Top Eurostar Destinations from London
See also: 2-Day Tour of Paris from London
Two-Week Self-Guided Itinerary:
Consider a day in Lille, followed by Paris, Brussels, Antwerp, and Amsterdam.
Time-Saving Guided Tours
If you don't have two weeks, consider taking a day trip or two to fit in more cities in a shorter period.
Whistlestop Guided Tour of Europe
This 11-day tour of Europe fits in Paris, Milan, Venice, Salzburg, Prague, Berlin, Amsterdam and Brussels
Of course, you could do the above tour in reverse. But there is also a great route south through Europe that ends in Italy. This is a great independent tour for spring when the tulips are in bloom in the Netherlands, then you can head toward warmer weather down south, passing through western Germany on your way to the lake region of Italy.
If you have more time, this tour connects with a tour that heads east to Venice.
See also: Brussels and Antwerp Tour from Amsterdam
Two-Week Itinerary: Amsterdam (3 days), followed by Cologne (2 days), as much time as you waste in the baths of Baden-Baden. You then have some options in Switzerland before arriving in Italy.
Alternatively, hang a left after Frankfurt or Baden-Baden and head to Munich, where you can pick up the Munich-to-Venice itinerary as featured below.
If you want to do a long and memorable tour, touring Europe's Mediterranean coast brings you to some pretty enticing destinations. This tour takes you from Valencia, Spain to Rome, Italy. You can do it in a car, but wouldn't watching the scenery go by from your seat on a train be a better thing to do? Remember that beach cities get crazy in August.
Two-Week Itinerary: Start in Valencia for a day if time allows, otherwise begin in Barcelona (at least three days). Stop in Figueres for the Dali Museum, before heading north for a day each in Narbonne and Nimes a couple of days in Avignon before spending a few days in the French Riviera. After this, it's Italy all the way - a couple of days in Genoa and a couple in Cinque Terre before ending in Rome.
This tour, dipping into Eastern Europe before returning to Germany, makes a half-moon route through some spectacular, old-world European sites, including Venice, Vienna, Prague, and Nuremberg. Like the touring route above, the distances between destinations are short, the train rides are spectacular through the alps, and you can fit it all into two weeks, although longer is always better.
Two-Week Itinerary: Three days in Munich, then a day in Nuremberg, followed by three days in Prague. Then head south via Cesky Krumlov (if partying with backpackers in a UNESCO-protected World Heritage Site is your thing) to Salzburg. After a couple of days in Salzburg, take the train down to Venice.
A possible extension is to swap Cesky Krumlov for Vienna, where you should spend at least three days.
Your tour of Scandinavia should hit the big cities of Helsinki, Stockholm, and Oslo and then balance that with a trip to the fiords of Norway. Don't miss one of Europe's most interesting train rides on the Flåmsbana, a special railroad built to descend from 866 meters to almost sea level.