that hire car insurance is right
if you're considering switzerland, you could also look at a few days in the mountains instead of one of the cities? there's quite a few mountain resorts that have railway stations and accomodation within walking distance. for example the lauterbrunnen valley is spectacular, especially if you can find somewhere to stay in wengen and the mannlichen cable car is open for walking. you should still be able to get the train to kleine scheidegg though. switzerland has some pretty spectacular mainline train routes
nuremberg is a really nice town, it's very "german", and the historical stuff is very interesting. the way they repurposed what is a pretty sinister part of local history and used it for motorsport is fascinating in itself, especially in person. berlin is really good too, it's a very lively city and schonefeld airport is very easily accessible on the train from the centre plus has flights to nice. you've got loads of great options there, germany is pretty easy to get around on the trains, and if you can book in advance (a week is normally enough) on inflexible tickets it's pretty good value. i've only ever been to the airports in munich and stuttgart so can't offer words of encouragement there!
you'll be heading into the nice/monaco area at the right time - the film festival starts a few days after you leave and that's when accomodation and flights get really expensive! it's also a very pleasant temperature too