Ms P, as has been mentioned, there is no single *correct* solution. All those things you mention are fantastic for getting students involved with Science and Technology.
Scratch is awesome because it is free and very easy to use. There are fantastic resources all over the web that'll help integrate it at just abut any year level.
Arduinos are awesome as they give kids the chance to see what is actually happening under the hood of most electronics. The software is very easy to use and there is so much example code already written, that students often just need to pick and choose parts of code that already works,and modify to suit their projects
LEGO Robotics are awesome as it gives kids a chance to see something actually move and interact in real life, not just through on a computer screen. It is a great way to make real a lot of concepts that can sometimes be difficult to visualise and grasp when they are just equations on a whiteboard.
Regards
Damien Kee