I have been a sucker for mountains since before I was a teenager. But you might prefer biking along the eastern coast of Italy, through the vineyards in Bordeaux in France, between cities in Holland, or down the Danube river. You (and your bike partners) need to find the right exercise level. And what is your main interest, except biking? Food? Wine? Photography? Architecture? Theater? Art biennials? Mountains? Europe has it all!
And be creative! You can start with three days up the Rhine river in Germany, then take the train to Bordeaux in France, next visit the film festival in Cannes, and finish it off in the Dolomites in Italy. Inter rail tickets will let you do that. I’ve bought several Inter rail tickets the last few years.
On this website are several short chapters describing various trips I have done. You can use them as inspiration for your own planning.
How to get there
Trains are the best way to go, at least if you travel through western or central Europe. Trains in the Balkans are very slow compared to Germany, France and Italy, so long distance buses might be a better choice through former Yugoslavia.
Another interesting choice is to split a car among say four bikers. While three are biking, the fourth have a day off and drive the car with all the gear, somewhat close to the bikers if there’s a problem. Equip the car with a roof rack that can hold all four bikes when going to A and home from B. I have never done this, but it sounds like a great option.
The best time to go touring in Europe is May to September, but there are a few options outside these months. One is Sicily which has decent bike temperatures in winter. Another is Albania, which is cozy in April. And Crete can be nice in February. I also believe southern Portugal is nice if you want to go touring in winter.
Try to avoid the peak summer vacation months wherever you go, since that is when you have to compete for hotel rooms with millions of European summer tourists. Another warning is around Le Tour in France, the Giro in Italy and La Vuelta in Spain. Check out their routes ahead of time, either to watch them bike, if you’re into that, or to avoid the insane crowds.
Close to Europe is Morocco. I spent eight December days biking across the Atlas Mountains in great biking weather. The year before my visit they had half a meter of snow in the mountains, so I was lucky.
Your packing needs to be adjusted to the day and evening temperatures of your destinations. Morocco was perfect at daytime, but the temperature dropped like a stone to +5° C as soon as the sun set. I froze quite a bit on that trip. No fun. Use a site like Weatherbase to compare the average temperature of a destination city with your own home town. For example, Marrakesh in March has about the same average temperature as Stockholm in July. Estimate the average rainfall using the same method.