Strasbourg to Salzburg, where to stop for 2-3 days?

You might want to move this post to the General Europe forum, since it covers 3 countries. Just keep in mind that Lake Constance will be quite warm in August, and you will also have lots of tourists to contend with, as German schools are out on vacation in August. And Germans (and most Europeans) tend to head to the beach when they're on holiday.

You might want to consider Augsburg, which is right on the way to Salzburg. It's a charming town but would not be as crowded as some of the other more touristed sections of Germany.

I personally might not make a stop on this 5-6 hour trip. While it is a "detour", Innsbruck (a small city, but definitely a city) is a fairly obvious idea, and unlike huge Munich, it has a "cable-car mountain" right "downtown" walkable. If I could get an opera ticket for the Seebuhne in Lake Konstanz, I would do that, but not everyone like opera, even James Bond Spectacle-sized opera.

IMHO, a scenic uncrowded place, no-car, is likely not to have enough for 2-3 days, even if you can find what is currently, on this board, romantically called "a village." Even in Salzburg, some in-town attractions can be a pricey cab ride, like Schloss Hellbrun. (Not questioning that Salzburg IS a must-do.)

I was just in Konstanz en route from Garmisch to Strasbourg and can recommend the Wildhaus Jakob. Normally I stay in Dornbirn on the south end of the lake as it's less crowded, but I was showing family around and they wanted to do the 24 hour Flea Market and Mainau, so that was more convenient. The Four Points Sheraton in Dornbirn is excellent; good location and nice hotel.

Since you're going by train though, the normal route from Strasbourg to Salzburg is via Stuttgart and Munich. You may want to look at Augsburg as a stop if you don't want to stay in Munich. The Altstadt there is quite nice (better than Ulm).