I don’t have a budget for robots at my school but I would get the Picoboards since they are much cheaper. The Picoboards are really great because it runs using scratch and can be a seamless transition into coding on a real scale with the language they are now comfortable with.
In this alternative unit, I would create different tasks for the students that way they have to problem solve and create a storyboard for what it is they are trying to accomplish. They would need to work in pairs so they can collaborate together to get different points of view and get the best programs possible. In my search for activities I saw several great websites offering lesson plans that I would use. Nebomusic is one of the websites that I found great lessons on. Youtube also has some awesome videos of tutorials!
The structure of the unit would include some intro days (setting up the board and learning what each part of the board is and what it does.) Then I would move into how to program those parts of the board (looking at how the code would control those pieces of the board.) Then I would transition into challenge days. In these days, The students would get time to work on different challenge I would give them and the groups could work at their own pace. When they are done with one challenge, they can move onto the next. The final project would be to enhance one of their programs and extend of some of the code they already had built.