There are some members who are quite experienced. I don't have as much travel experience as a civilian. Have to change that.
Fortunatelt the international language for travel is English. Try to get train schedules in advance, many Embassies and Consulates from various countries here in the US will send you train schedules and other stuff as part of their Tourism promotion. Just have to call or write.
I believe their is a an over night ferry from Italy to Spain. Naples to Barcelona? Can't remember exactly. The the train from Barcelona to Nice france, and Nice to Paris. Course you need to take the high speed to Madrid and i recommend Toledo, and of course Cordoba. I lived in Spain for three years and know that better. If you run into anyone Irish. Attach yourself, they will know where there is an Irish bar, and you can eat.

Seriously I found two in Sofia, Bulgaria, Ireland exports nationals like none other. Wandering feet I suppose.
Three weeks? You gonna wish you had three months. The back pack is certainly doable. Just pick up travel clothes that are wash and wear. Stuff you can scrunch up in a ball and just steam out.
Cell phone, or commonly called a mobile; these are cheap. Pick one up at a kiosk, no contracts just pay as you go. Then you can reach any new friends you make to meet up for some chow or reach emergency services.
A note pad and a pen are helpful too. Even if you don't speak the language you can draw or write down numbers.
July and August are huge holiday months in Europe. People rent houses and take month long vacations to one oceanside resort of another. So If you stay away from the coasts and visit in land cities you won't have to fight your way through hordes.
Deodorant. Totally unheard of.
Back to clothes. Buy and wear local. People won't be trying to steal you fashionable American clothes and you will attract less attention from pick pockets and purse snatchers.
I will try to think of more.
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