Luzern is great. I live less than an hour away, I visit it often, and never get sick of going there. You have a lot of options from there. It's not too touristy, but still caters to tourists quite well. There is the transportation museum, and the Rosengart collection that has number of Picassos, if you're into that. Nearby Pilatus is a very nice trip on a clear day. Depending on what time your flight leaves, you can stay in Luzern and go direct to the airport in Zürich.

The train from Luzern to Interlaken you want to take is the one that goes over the Brünig pass. It's kind of slow, but it's a nice trip. Reichenbach Falls is not any easy trip from Luzern as the trip over the Brünig takes a good bit of time.

The train up the Jungfraujoch is best done early in the morning on a clear day. With a few clouds, it's okay, in overcast it's not worth doing it.

Brig is okay, but on a 7 day trip, I wouldn't stay there. Chur isn't that exciting either. Do an overnight in Davos and spend a bit of time in Tirano on your way there. Then go from Brig to Lausanne and spend the night in Lausanne. Go the next day from Lausanne via the Golden Pass to Interlaken, or via Bern if you're in a hurry.