A couple of things come to mind that take time for the kids to prepare and execute (which helps pass the hours!)...
1. If you have a video camera, have them make a movie and then have a movie viewing party where you make popcorn, soda's or juice etc... You can make home made theater tickets, and serve as the movie ticket person, and concession person. The kids can set up the movie viewing room prior to the showing with tons of pillows, or a bed mattress on the floor, or piles of blankets, air mattress etc... Once their movie is over, you can watch a rented flick for fun too.
2. If you have a digital camera and they can print the pictures on paper, you can give them each time with the camera to take pics of whatever interests them. Then you can have them print their pics, and display on construction paper, or in old pic frames and decorate. You can have a gallery party where they display all their work and get small prizes (which can be homemade or recycled). You can take it as far as dressing up for the showing, having trays of appetizers, playing music in the background, or inviting neighbors or family to come over. If you really want to jazz it up and make it more fun for the 14 and 11 year old, if you invite family over for the party, possibly they can "buy" some of the artwork.
3. Top Chef competition.
4. Not sure the genders of the kids, but if there are a lot of girls they can do a spa party. Give manicures, pedicures, do hair, makeup etc... and then do a fashion show.