My wife asked me to come in and helped teach a lesson in her class. Here is my gameplan:
Teaching targets: algorithms, boolean operators, and loops
I will start by introducing them to a game they've all probably played before: Guess the Number. I will let one student try to guess a number between 1 and 100 and tell them higher and lower. A few game in, I hope they will develop the optimal algorithm (binary search). I may then throw in 1000, pretending it will be so much harder (but it won't be if they have a good algorithm.
After they get the procedure down, I move to generating code to guess the number. They will author in Snap! a bit of code that will take a number as input, then tell them hi or low. Then they will develop a bot that will find the number using the binary search logic they developed above. They can even have bot competitions to see which one can find it the fastest. This will probably be enough of a challenge for at least one if not more class periods.