Thanks for the compliments.
A lot of interesting things could be done with the Web Portal, since it serves as user interface of sorts; it can display HTML, execute JavaScript, and run CGI on your reader. So you could either make a Sudoku you can play on your reader or even build for example a sync feature that grabs EPUBs from your Calibre Content Server (or even displays an interactive selection of books).
I'm not sure if I'll actually make something like that though (I don't use content server and I'm perfectly happy with using PC to get new books on the reader), and not sure what else to do with the possibilities...