I have an 11 hour layover in Copenhagen. Is that enough time to leave the airport, go see the city and get back in time for my flight to Amsterdam?
Yes! That is plenty of time to see the city.
The Copenhagen airport has easy access to downtown via direct metro train - I think it is probably one of the easier places to do something like this.
15 minutes is all it takes to take the train from CPH to Centraal train station. You have plenty of time.
I'd suggest the metro instead of the train. The train takes you to the central station at the edge of the city centre while the metro takes you to Kongens Nytorv, in the middle of the city centre.