Idleness is a characteristic of being inactive. Your idleness over the summer means you really didn't do much — besides snoozing in a hammock and sipping lemonade.
Anything that's not in motion has the quality of idleness, whether it's a car sitting in a parking lot with its motor running or a lazy dog lounging on the couch. This noun is often used to mean laziness, particularly when someone's not doing what they should: "His boss wasn't happy about his idleness at work." You can also use it in a more positive way: "The idleness of lying on the beach all day is my favorite thing ever."