Things to do outside with young kids?

I am 14 years old and about twice a week I babysit two girls ages 3 and 5. I babysit during week days so the dad is at work and the mom is usually just running around town getting things done. But the days that she wants to clean the house without the kids I'm not really sure what to do with them. They're bored of the usual pool, bubble, chalk, tag ect. I cant drive them to like a park or anything since I don't have a license so I need something to do in the back yard. Any ideas?