Canvas with form
Being able to run Processing.js sketches on a web page is really exciting, but I wanted to be able to add GUI elements (like buttons and drop down boxes). Basically, I wanted users to be able to interact with my graphics using a HTML form, but searching for a tutorial turned out to be fruitless. However, I was lucky enough to find a website that talked about sharing variables between Processing.js canvases, which helped enormously. You can find that website at http://bocoup.com, and it's well worth a look.
Being able to communicate two ways between Processing.js and HTML forms opens up many exciting possibilities for dynamic and interactive web pages. Hopefully this page will help you make some of your ideas a reality :)