Train site: http://reiseauskunft.bahn.de/bin/query.exe/en
If you want to save some money, here is a perfectly legal way to buy the ticket. I'll assume you are two people.
Buy a Baden-Wurttemberg Ticket for 2 people costing 28 EUR. You have to pay an extra 2 EUR to buy from a ticket window rather than a ticket automat.
Buy 2 local tickets from Kehl to Strasbourg for 4.10 EUR pp.
Here is the quickest way.
Take the 10:42 departure from Schluchsee arrive Freiburg at 11:48. This is a Regional Bahn.
Change to the 12:03 departure from Freiburg arrive Offenburg at 12:53. This is a Regional Express.
Change to the 13:05 departure from Offenburg arrive Strasbourg at 13:34. This is a Suedwestdeutsche train.
There are hourly departures, the above is just the quickest. Others take longer because there are longer times to make connections.
So the B-W ticket gets you as far as Kehl, which is across the Rhine from Strasbourg, and the local tickets cover you from Kehl for the short trip over the bridge to Strasbourg. You don't have to get off the train. If you just buy local transport tickets from Schluchsee to Strasbourg, it will cost you 61.40 EUR, instead of 36.20 EUR using the B-W Ticket. If you buy tickets without checking the "Local Transport only" button, you will get there a little quicker using an ICE train between Freiburg and Offenburg, costing 74.20 EUR.
All of these tickets can be bought at the time of travel. No price advantage to buying in advance.
In Strasbourg, take a boat ride through the canals of the old city. Check out this site: