To keep tweens and teens on their creative writing toes this summer, try challenging them with a set of fill-in-the-blank comic strips.
I copied some for my children off of Yahoo's comic page to my computer (if you were planning this for a group setting, you would need to get copy write permission). Then I combined them onto a single sheet in "Paint", and used the eraser to blot out the original captions, so the children could fill in their own words to go with the pictures.
I had planned on saving it for later today, but my older girls (ages 14 and 16) discovered, and filled theirs in, during a late night kitchen raid, last night already.
They got quite a kick out of seeing how their dialogue differed from each others, but still managed to fit with the pictures.
I crammed quite a few comic strips onto one page - fewer strips per page would make for larger word bubbles, and might be easier for slightly younger children to fill in.
It might also be fun to leave the words, and erase the pictures, for the artistically inclined.
Or, for children blocked by empty word bubbles, you might start off with a matching exercise, by cutting out a set of word bubbles for each blank comic strip, so they could mix-and-match the original words with the pictures.