EDIT: Ooops, I posted little too late maybe it's gonna help someone else

If you choose to travel to Vienna by train, don't forget that you can add public transportation in Vienna for one day to your ticket for just 3,60 EUR. Also, your ticket would be valid for both routes - to Hlavna stanica and Petrzalka. You can choose to travel outbound from Hlavna stanica and return to Petrzalka, if you'd like.

If you'd like to eat close to Danube, I'd definitely go for some restaurant in Eurovea mall - they are all located on embankment and most of them is good quality. Alternatively, I like restaurant Ako doma which is nice, have really good food, mostly good staff and nice prices. But it's located little out of hand.

I wouldn't use taxis unless absolutely necessary. They will try to overcharge you and don't deserve your business. Buses run round the clock, 365 days a year*. Special fare is payable on night services, but if you have 24 hours ticket (or ticket with longer validity), you'll be fine. You can't buy ticket in bus on any intra-city service.

(* well, there is exception, we don't have bus services for 2 hours a year - that's 2:30-3:30 on Christmas day and one hour when we switch to winter time, I guess that won't affect you )