OK, so the archetypal human story is for a guy to go out and impress the girl with some wonderful achievement like slaying a dragon or building a monument or conquering neighboring tribe.
This being enlightened 21st century on SO, let's morph this into a:
StackOverflower performing a feat of programming to impress a romantic interest.
There are two ways to do this:
Technical achievement:
Impressing a person with suitable background/understanding of programming with actual coding powerss you displayed.
A dumb movie example would be that kid in "Hackers" move showing off his hacking skills in front of Angeline Jolie.
Artistic achievement:
Impressing a person with a result of running said code, whether they understand just how incredible the code itself is.
An example is the animated ANSI rose (for a guy who actually wrote the ANSI code)
This question is only about the first kind (technical achievements) - e.g. the person of interest was presented with impressive code/design that (s)he was able to properly appreciate.
Rules (what doesn't qualify):
The target audience must have been a person of romantic interest (prospective or present significant other or random hook-up). E.g. showing your program to your sister who's also a software developer doesn't count.
The achievement must have been done specifically with the goal to impress such a person.
However, it is OK if the achievement was done to impress a generic qualifying person, not someone specific. Although... if you write code to impress girls in general, I'd say "get a better idea of the opposite sex"
The achievement must have been done with the goal of impressing the person. In other words, if you would have done it without romantic interest's knowledge anyway, it doesn't count. As examples, the following does not count: programming for your job. Programming for a coding contest. Open Source program that you'd have done anyway.
The precise nature of the awesomeness of the achievement is somewhat irrelevant - from learning entire J2EE in 2 days to writing fancy game engine to implementing Python compiler in LOGO. As long as it's programming/software development related.
The achievement should preferably be something other people would rank highly as well. If your date was impressed with your skill at calculating Fibonacci sequence without recursive function calls, it doesn't mean most developers will be. But it does mean you need to start finding better things to do on dates ;)