Like other experienced teachers, I've taught the same concepts and skills to students so many times that I've developed a huge repertoire of activities that I can build into my instructional time. I have activities that provide more scaffolding if the kids didn't really seem to understand the lesson, and activities that will extend student learning if what I taught was too easy. I have activities that will take a long time if my lesson ends early, and activities that can be done quickly if my lesson takes too long.
Math is a subject of great divide: some kids love it, and other kids hate it! You can help make math fun by providing variety, engaging materials like fun math worksheets, and academic support.