One that always works as a intro to other things is picture->story generation:
Draw a simple picture on the board. (For reasons of the long lost origin of this activity, I always draw a simple sail boat on the ocean--a wavy line--with a stick figure boy on the deck.) Elicit a description. The class decides what subject-verb-object, but you can prompt them by starting the sentence yourself. Write up whatever they suggest, leaving blanks for missing grammar and underlining bad grammar. Signal heavily that some correction is needed.
Once the first sentence is writ, correctly, alter the picture. (I usually set the boat on fire.) Elicit a sentence to describe the new situation. Repeat the grammar procedure.
Repeat until exhausted or the story finishes. Then move on to something else. At various times I've moved on to a story-telling tenses lesson, a modal verbs lesson that needed some initial scenario to talk about, and some other thing I forget, something about interpreting pictures and discussing action. As an introduction to other things, it's often good to stop in the middle of the story (or what you judge to be the middle, where a turning point is needed) and go on to the rest of the lesson. This works well for classes on giving advice--you get the class to go into groups and decide how the story should end and then produce advice for the characters.
If you have an imaginative class, you can do it without the pictures--start by saying, "how many people in this story?" If you have a class that can't cope too much with making up plot lines, use the pictures and over the second picture write "unfortunately", and over the third picture write "fortunately" and so on, alternating fortunately and unfortunately. Soon enough they get the idea and start making storylines.