CS Unplugged has some good active learning stuff, and all relating to computer science.
Computer science is as much about computers, as poetry is about pens. Therefore only use the computer where it adds benefit. CS unplugged, is an initiative to teach computer science, without the use of computers. It is mainly focused at primary schools, but I have used it in senior school. (If they are successful at teaching all pupils at primary school, then we will have to find other activities.)
I have use some of the activities on cs unplugged. Not after lunch, but just because they are good activities.
I have in my teacher pack some basic materials that allow me to teach a range of these activities.
An easy one that takes little preparation, except purchasing a pack of playing cards, is: sorting algorithms.
One pack of cards can be used for 4 groups of 2 to 4 students (so up to 16 students), but sometimes you want them to use more cards, see below.
Separate out one suit of cards for each group. If they don't know cards well, then remove J,Q,K. And shuffle cards (or get students to do it).
Now as a class get one pupil to sort cards. Watch what they do, and identify the algorithm that they used [usually insertion, or selection]. Tell them that they just did … sort.
Then break them into groups, and get them to experiment, what algorithms can they find. Tell them to sort a whole pack, this it to put enough pressure on them, to have to find a better way to do it.