There's some good advice in here. I just got back from 2 weeks spent in this area, most of it in Slovenia. It's now my favorite country in Europe. Stunning natural beauty. We drove up to Berchtesgaden/Salzburg for a couple of days, and the Sommerroldenbahn (toboggan) is fun for anyone at any age. Also, there's the Ice Cave (Eisriesenwelt, about 30 minutes south of Salzburg) and the Salt Mines (Hallein has the largest one, I think). I spent a day and Trieste and unless you were looking to eat some Italian food, it's pretty skippable, to be honest.
I cannot sing Slovenia's praises enough, though. The town of Bovec is an outdoorsy person's dream. Rafting, hiking, swimming, waterfalls, it has it all. Croatian Istria isn't far, either. Look up Predjama Castle, Skocjan Caves, Lake Bled and Lake Bohinj to see some of the things we did in Slovenia.
Eh, as someone who was taken to Paris as a 12 year old, I hated it. The Eiffel Tower was cool for 30 minutes, as was the Arc de Triomphe, and as a plane nerd I loved the museum at Le Bourget, but honestly there was nothing else I remotely liked about Paris. I'd hold off until 16+ for Paris when they have a little more appreciation for the history.