Most springs I teach 15-251, "Great Theoretical Ideas in Computer Science." The course usually has about 200 students, which makes it a ton of fun. Over the past 3 years, the first assignment has been a "Puzzle Hunt," in which groups of four students compete to solve a bunch of puzzles and run around the campus. The first team to finish usually wins iPods for everybody.
This year's theme was "Save Gates!" The story was that a rogue professor from the math department was planning to sabotage the new computer science building (Gates), so the students had to foil the secret plans of this envious enemy.
I have to say I had a lot of fun making this assignment.
You can see some of this year's puzzles here: http://1982087.com/ (for those of you who are curious, this number is the 15251st prime in this sequence of numbers; finding this number was the first step of the hunt).