We hold an "astronomy camp" or Astrocamp for short every summer here at Wyoming. There are a couple of different groups that attend. I'm helping out this year and have 20 bright and interested junior high kids for "science fiction writing" one evening this week.
I'm still planning the activity, but basically at this point it'll be a combination of lecture by me on some things (size scales, speeds, etc. of the universe, the business of writing), discussion about some specific topics, some movie interludes and discussion ala www.badastronomy.com, and some writing exercises.
I think it'll be a lot of fun. I'm open to suggestions, too, since I haven't worked with kids this age much. Drop me a line at mbrother@uwyo.edu.