This app was mentioned in 2 comments, with an average of 1.50 upvotes
I've got another tip for you, the Wiener Linien have their own app where they offer a special day-ticket. Here is the link to the App in the Google Play Store
The day-ticket costs €5,50 and it's valid from midnight to 1AM the following day. If you have one day where you want to explore something else than the first district this might be a solid option for you. You can buy it through the app whenever you need it.
And as all the others already suggested, don't take the CAT, take the S7. It's much cheaper and takes only a few minutes longer. Don't forget to buy a two-zone ticket, because you start outside of "Zone 100".