My mom, sister and BIL are all teachers, who have done a large variety of things during the summers over the years.
When we were younger, my mother tutored out of the house. She generally only needed to work a few hours a day, and didn't need to worry about child care for us. As we got older and could stay home alone, she often taught summer school (she's Special Ed certified, so usually in high demand for summer school). Now she's on an extended contract, and only has a week or two off in the summers.
Sis and BIL generally switch off with one of them caring for their kids and the other working. He has run sports camps, she has occasionally taught summer school or run less formal summer enrichment classes at her school. This year she'll be teaching Adult Ed classes.