Strasbourg for 6 hours

Yes, visiting the cathedral is worth it, particularly to see the Astronomical clock. If you are standing facing the front of the cathedral, the TI is just to your left. They have an audio guide 1 1/2 hour walking tour. With the limited time you have, concentrate on the central historic district. Check out the Tourist Office on this link.

In Strasbourg you must dine on Choucroute Garnie and for one of the best, from the Cathedral, walk down and cross the River Ill. The restaurant Au Pont Corbeau is located at 21 Quai Saint-Nicholas.

Take the boat ride. Walk around Petite France

We enjoyed typically Alsatian meal (dinner) at a restaurant called St. Sepulcre. Near the cathedral, to the north. 15 Rue des Orfèvres. They’re open for lunch as well as dinner.

if you couldn't find Schnapps in Speyer you didn't look in the right places. You won't find it in France.

So you have 6 hours in Strasbourg, on a rainy Sunday. The cathedral is problematic unless you intend to attend services. Mass is at 0930, 1100, 1800. You can walk around La Petite France, but that's best on a clear day or at night. You may want to look in at Galleries Lafayette for Christmas stuff, but they're closed Sundays. As are most of the shops. What you want is a Carrefour superstore or a Tedi; look for them in your next town.

Palais Rohan is probably your best bet, right across the plaza from the Notre Dame.

Schnapps can mean different things, but if what you want are the strong clear brandies made out of different kinds of fruits, the phrase you want in Alsace (or elsewhere in France) is eau de vie, and that should not be hard to find in Strasbourg as they are an Alsatian specialty.