Vienna's famous outdoor food market: Cheese, fish, olives, falafel, fruit & veg, etc etc, plus cafes and restaurants too. You can get a legit light lunch from all the free samples you'll be offered, but beware, they're wily businessmen around here! It's particularly worth being here on Saturday mornings, when there's a substantial antiques / fleamarket next to Kettenbruckengasse station. Neighbouring streets on the right side (heading towards Karlsplatz) also house a smattering of Japanese / Korean joints, if you've been craving ramen / sushi / kim-chi.
The Naschmarkt's open Mon - Fri 06:00 - 19:30, and 6 - 6 on Saturday. Closed Sunday. It starts around Kettenbruckengasse U-Bahn station (on the U4 line), and continues east towards Karlsplatz and the old town.
Getting there: From the hostel, take the U6 U-Bahn line from Westbahnhof station (direction 'Siebenhirten') to Längenfeldgasse, then change to the U4 (direction 'Heiligenstadt') to Kettenbückengasse. Or walk there from the hostel in about 25 minutes.