One activity using Scratch that I have run with Year 3 is to make a dichotomous key. We started by making a key for monsters. We scaffolded the process with a presentation which can be found at this link.
The presentation started out quite simple, and got more and more complex.
At the end, once students were comfortable with the flowcharts and the idea of how a dichotomous key works, they were given the Scratch code for the simplest key (for only two monsters).
And they had to recreate this in Scratch.
Once they had done this successfully, they needed to use nested if/else statements to create more complex dichotomous keys. Once they had completed this, they needed to use Scratch to create dichotomous keys for animals.
The completed Scratch project can be seen here.