regular imagination games are fun.
One I always do is, if the parent(s) will be home before bedtime, suggest that you put on a play. It's not gonna be shakespeare and it probably won't even turn out to be a play, to be honest. but it will keep the kids busy. I've had kids start off with the "play" idea (they wanted to do "snow white") and spend DAYS walking around pretending to be the huntsman and Snow White.
or, if you'll be babysitting while it's dark and if the little ones aren't afraid of the dark, get a flashlight and some black construction paper or a heavy brown paper bag. cut little teeny holes in your paper thingy and rubber-band it to the flashlight. turn the flashlight on in a dark room, and instant starry night! I did this with kids the same age as the ones you're dealing with- we spread out a blanket (that I had brought, so I knew it was clean and okay for the floor) on the kitchen floor, I made a "picnic" dinner, and we had a "starlight picnic" on the kitchen floor.
making cards for peole tends to be a hit. It can be for anyone- mommy, grandma, the cat...
Do you know what they're into? cars? dinosaurs? spiderman? you could print out pictures pertaining to their interests and let them have at it with the crayons. It's always more fun for a boy to color a picture and be able to tell you, "that's a velociraptor" than it is for him to color any old generic coloring book picture of an ugly little girl flying a kite or a hat with flowers on it