The 'LeichtSinn' is a really lovely place serving excellent, healthy food. They offer a range of vegetarian dishes, including lasagne and curries, and they have a very nice salad bar. Almost everything is freshly prepared and they try to offer organic food where possible. I've heard that they are very happy to prepare delicious vegan dishes on request. The atmosphere is laid-back and staff is very friendly. It's also a nice place to have a coffee and something sweet; their choice of sweets is rather untypically Austrian, though (they'd have shortbread and brownies, for example).
I especially recommend their home-made ginger-lemonade - it's delicious!
Cons: it can be crowded during lunchtime