If your students are eager to keep up and stay in shape with new topics, suggest puzzles, like from Martin Gardner books, or Godel, Escher, Bach by Douglas Hofstadter.
If looking for theme based studies, recursion is always a good one, links to fractals and phi. Puzzle: "my whole is to my longer as my longer is to my shorter, so what's my longer's length if my shorter's is one?" (provide a picture?).
But without getting so numeric, consider geek lore: GNU stands for GNU is Not Unix, so the G stands for GNU, a kind of "not joke". Likewise the Y in YAML stands for YAML, which Ain't Markup Language.
Speaking of markup languages, XML and XHTML remain hot, along with JavaScript and the DOM. I was just running these by some gnu math teachers today in fact, as more topics in ~M!.
Here's an idea: obtain Periodic Table data in YAML, either as open source or from scratch (and then share it?), or how about graph the bones of the body (which connects to which?).
The point of these projects is not to create unnecessary tedium or drudgery. Let students take a lot of time learning about the elements, about bones, including from first person experience in the great outdoors. Nature guides a big plus sometimes -- your student might become one?
Recommended chemistry readings: Uncle Tungsten by Oliver Sacks, Isaac Asimov's stuff (a chemist and fabulous writer). Biology: Lives of a Cell by Lewis Thomas. Mathematics: surf the Web for stuff on phi, fractals and Fibonacci numbers. Visit Koski's Kiosk?
Make sure your students are well briefed on bookmarking, maybe suggest Delicious as an option? I've certainly gotten value from that service.
Note to students: avail of work freely shared, while showing respect to your peers. Give back. Cultivate generosity. Study potlatch economics? Start a new blog?