Moving on with Google Apps engine. After I've ported some logic into Java, now I'm looking on how to render some output.
There are nice introductory tutorials about Servlets and Java Server Pages but these are a bit outdated. Nonetheless, they represent an excellent introductory material. Then, there are intermediate and advanced tutorials, with material available in PDF. There is more than enough material to get you started.
I have found the level where I got stuck and am going to enjoy some reading about templating and scripting.