The Christmas Markets at Wolfgangsee include Sankt Gilgen, St. Wolfgang and Strobl. I recommend seeing all three. You take the 150 bus from Salzburg to Stobl, then the Christmas Market ferry to St. Wolfgang and then ferry again to Sankt Gilgen. From Sankt Gilgen, you take the 150 bus back to Salzburg. If you have time and if the weather is clear, I recommend taking the Christmas train up to the top of Schafberg from St. Wolfgang.
Each of these three connected markets have a different theme, which is lovely to explore. If you don't want to visit Strobl, then get off the 150 bus at St. Gilgen, ferry to St. Wolfgang and then ferry back to St. Gilgen. The 150 bus doesn't go to St. Wolfgang and a change is required, which is time consuming.
In Salzburg, be sure to visit the Christmas market at Hellbrunn Palace, probably the best Christmas Market I have been to (and I've been to a lot).
As for lunch/dinner recommendations for your day on the Wolfgangsee, you will want to just eat food from the markets, right?