I'm going to be in Copenhagen in September, and was wondering if any attractions/shows/etc would require advance purchases. I haven't seen anything yet, wondering if I missed something.
I found the Copenhagen Card very useful. The web page has a calculator to help determine, based on your plans, if it will save you money. I preordered it to pick up at the airport ( received a voucher online to show) , and used it right away for the (so easy) train to the city. But you can buy it on arrival. A couple of times it saved me having to go buy a ticket, and there was nowhere I went it didn't cover admission. Check out Trip Advisor's forum, too, with this question.
Attractions no. Shows probably. But of course it depends on the particular show you are interested in.