What to teach after Scratch?
Take a look at Processing.
It's tour de force is graphics, animation, and visual manipulation. It runs under Linux, too.
Processing is an open source programming language and environment for people who want to program images, animation, and interactions. It is used by students, artists, designers, researchers, and hobbyists for learning, prototyping, and production. It is created to teach fundamentals of computer programming within a visual context and to serve as a software sketchbook and professional production tool.
If Scratch is starting to get a bit limiting, but they're not ready for the hardships of text-editor coding, take a look at Scratch-derivative "BYOB" (Build Your Own Blocks). Seriously, it turns Scratch into a grown-up programming environment with functions (and hence recursion), data-structures, multithreading and everything!
There's also Panther but I was less impressed by it (creating new blocks in Panther seems to require coding their function up directly in Squeak, while in BYOB you can just build them in the usual drag-n-drop Scratch style).
How about lua?
There is nice graphic "engine" called LOVE which is fully programmable in lua. It has nice documentation and it's not very hard.
There are also several other similar engines using lua:
- Novashell
- Verge
- Luxinia
- Agen
There was another 2d engine, but I can't find it at the moment, it was similar to LOVE, but with a little different approach to things.
I would recommend LOVE for starters as it's very easy, has nice tutorials and most importantly you can do nice stuff right away.
Also lua is commonly used as game scripting language. For example all addons for World of Warcraft are written in lua, in fact all of the interface is written in lua. It means that it's very easy to find answers to game related questions in lua. Also if you happen to own a game which uses lua as scripting language, you could easily add your own stuff to it.
I wrote this from game perspective, but there are quite a lot projects which use lua as scripting language.
You could also try python, but it doesn't have so good out of the box, ready to use and easy to learn/understand tools.
Also here's a link to lua manual.