Everyone who goes travelling has a different experience, and there's pretty much no way you can screw this up and come back regretting it. No matter what you'll do, you'll have a blast. But here's a few things that might help smooth your way.
Know your budget, and be prepared to exceed it. Western Europe is expensive, and if you don't have a handle on how much money you should be spending on a daily basis, you can drain your bank account very quickly, or might find yourself avoiding things just to save a few bucks. Math it out as much as possible beforehand so you've got a ballpark idea. I would suggest a EUrail pass because, although it's a pretty hefty outlay before your trip starts, it takes some of the pressure off of travel decisions once you're there - particularly when you're in Germany or Switzerland where trains are damned expensive.
Bring a small, foldable waterproof case with you. MEC sells some nice seal line ones. They are invaluable when traveling along in beachy places, as you can put your cards, phone, and/or camera in them when you go to the beach, and then stow them in your board shorts pocket when you go swimming, leaving nothing on the beach for you to worry about. Also, buy board shorts with a pocket.
Learn some local language, but not too much. I've found you can get by anywhere in the world with about 30 words and the numbers 1-10. Once you know where you'll be spending most of your time, do a bit of research into that local language. I wouldn't bother with German, Swedish, Dutch, or Norwegian though - most of them speak English better than you or I.
Mix up your cities and your nature experiences. Going from city to city to city is a bit dazzling, and you'll probably prefer to split up major cities with quieter experiences.
Don't rush. Three months is not enough time to see all of Europe, and you have to resist the pressure to try. You will see some of Europe, so just try to identify what parts you're most keen to see, and be flexible when opportunities arise on the road.
Connect with people. Traveling alone becomes tedious rather quickly. Meet people, travel with them if possible. Hostels are great for this.
Book your hostels in advance. You're going in summer, and it's stressful looking for a place to stay while the sun is going down and you're trekking around an unfamiliar city with your backpack (also, buy a good backpack. MEC has excellent ones that split into two - a main pack and a zip on smaller pack - that are brilliant for traveling). In my experience, hostelbookers.com is the best hostel site (oh yeah, also have a wifi device with you - iPod touch might be ideal, but that's if you don't want to spring for a wifi enabled local phone, or rent a phone for the 3 months, which apparently is possible).
Insofar as places you should not miss, I would suggest Berlin and Interlaken/Gimmelwald are among the most fun places you can go in Europe. Rome is also fantastic. But there is no shortage of cool places to go and, again, rushing them is not the way to go.
Also, if you're there during Midsummer, you should be in Sweden. Or so I've heard...
Whatever you do, you'll have a great time. Don't stress it, but don't leave it totally unplanned either. Unless you're filthy stinking rich of course.
|