Google it. Find/make a list of (free) "fun" ideas. Make sure there are plenty of choices (30?) : Read a book, play a board game, paper airplanes, organize a photograph album/scrapbook, color a picture, research a favorite topic online, practice math facts, write a play, clean your room, organize the linen closet, organize a free "toy trade" garage sale with your friends, write a letter to Grandma, have a Kool-aid stand ...
I heard of one mom who put "take a nap" at the bottom of her list -- and if the kids didn't choose something before they got to the end of the list they had to do that!
Have a box of things to "create" with -- various papers, tubes from paper towels, pipe cleaners, bits of string, markers, stickers, chalk, ... Or a costume box.
There are oodles of books such as "how to have fun without the TV" or "what to do when you're bored" or "games children play" -- make a weekly trip to the library and check some out.
Make rules: Each hour spent on something else = 1/2 hour of TV or video cames.