As someone who freely traveled Italy during Bein HaZmanim, as well as long stop overs is Madrid, Barcelona, and Zurich, I'll say that most 21 year olds can manage on their own. That having been said, ending up somewhere with 0 prep time that doesn't speak English as a first language could be difficult, especially without a laptop or tablet. When I got rerouted unexpectedly through Frankfurt on my way back from Israel once, even though I had a few hours where I technically could have explored, I didn't want to do it without having looked into any info about the area, transportation etc.
I also sympathize with people that feel Paris is particularly dangerous for Jews (though I personally don't agree with that).