Dan M
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Only if you have the Vienna Pass, because it's free!
Above all, don't book in advance with prebooking sites!
We had prebooked the long cruise for around 92 euros, but when we went to the departure point the day before (to check it, as it wasn't clear on the prebooking site), we noticed that seniors had a reduced rate: 72 euros.
This wasn't listed on the booking website!
So we canceled our reservation and bought it again at the reduced rate.
As for the cruise:
Well, it was boring and we were disappointed; it wouldn't even be worth half the price paid (which is why I mentioned the free cruise with City Pass before) .
Unless you want to have lunch there, of course.
Still, if you want to take a Danube cruise, I think the short excursion is good enough and cheaper.
The thing is, you don't see as much of Vienna as we hoped and actually you see a lot of the Danube, which in itself is beautiful, but as I wrote, not exceptional at all nor stunning... relaxing, quiet, the same all the time, which is why 3-4 hours is way too long.