Vienna in early September, any recommendations

Hi everyone, looking at treating my girlfriend to a weekend in Vienna in early September and was hoping for some cool places to eat and activities. It is a surprise for her but flying out on the 4th September in the morning from London, so nice walk on the Friday afternoon somewhere and looking for dinner recommendations in the evening.

For the Saturday, booked the concert at Palais Auersperg with dinner, so something cool to do on the Saturday morning, great places to have lunch, etc. Then flying back on the Sunday afternoon.

We're both very active, we love adventure and trying new things. Anything to avoid as well?