I was a nanny for four children ranging in age from 1.5 to 7 for almost two years. The number one thing you can do is take them somewhere and DO something. Kids, by and large, love to explore.
Is there a Zoo or a Children's Museum near you? A beach? A park? Those would be my first choices.
If those are all out of the question then I'd try to have some projects around the house/apartment for them. Arts, crafts, etc. Lots of hobbie stores will have sections for things you can do with kids.
If that's our of your budget you might just be able to go for a walk with them. Depending on whether you live in a walkable city or not. If there are fun and interesting people, places, buildings in your area they might enjoy just being outside with you.
And my last resort is a movie, either in a theater or on DVD.
