Make sure you use Cabify (download the app from the App Store) or Uber. It is not safe to use taxis from the street.
All places accept credit cards, but make sure they don't take your card. Have the waiter bring the CC swipe reader to the table. It is common practice in Lima, they won't look at you weird. The Peruvian currency is the Nuevo Sol $1 USD is approximately S/. 4 soles. Also, many places accept $US dollars so don't worry to much about getting soles.
For those of you who have T-mobile, you get free internet in Lima and Cusco. Just activate your roaming when you arrive at the airport.
Lima is a relatively safe city, and Cusco is too. Just keep yourself aware of your surroundings and you'll be fine.
In Lima the average temperature will be between 62 to 72 degrees Fahrenheit. In Cusco the average temperature will be around the 60's however it can get sunny and with all the walking, you may want to bring light clothes.