Vienna attractions - prebook or not?

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I'm off to Vienna soon and am planning to prebook Upper and Lower Belvedere - any other attractions that should be prebooked to avoid disappointment? Or is it OK to turn up and pay on the day?

I probably would if you’re going over Easter. I took my son interrailing last year and didn’t book anything (memories of interrail ling in the 80s and 90s) - we found we had to be really creative about what we saw and did. (Didn’t go to Vienna, but found this to be pretty universal in major European cities)