Mindstorms used to be great, but compared to Arduinos and the whole ecosystem that sprang up around them, it is just too expensive, too proprietary and too limited. Here is one example: http://www.seeedstudio.com/depot/shield-bot-p-1380.html a pre-assembled $69 robot where you plug an ordinary Arduino Uno or compatible board, with a chassis full of possibilities for expansion using off-the-shelf components. For block-oriented programming there is now Scratch which is free software and works with Picoboards (also open hardware) and Arduinos.