I was just in Copenhagen 3 weeks ago, and first I must say that I LOVED the city! To second what most people have said, you will need an adapter, but most electronics nowadays automatically convert. So you likely won't need a converter, but double check your device. Also, the hostel we stayed at had USBs in the room, so I didn't even use my converter. Our hotel in Sweden had USBs too, so double check if yours does.
Credit Cards - In short, I used mine everywhere, and never needed a pin. At the subway station, the machines told us to enter a pin, but I just hit the green button and the charge went through. Sever people told us we'd need a pin and didn't want to run our cards, we tried explaining we didn't have a pin, but that didn't work. So we just told them we had a pin, they ran it, and the card went through just fine (without entering a pin). Just be prepared to sign a paper receipt every time.
For cost - Prices are high, but not nearly as bad as I expected. I bought a few days worth of breakfast at a supermarket and ate lunch/dinner at food halls or cafes. I was traveling on a budget and was able to eat most days for $30-$40 total. A cup of tea was $5, my shrimp/avocado smorrebrod at Torrvehallerne was $15, my cinnamon roll at St. Peders Bakery was $4.