As in many other things, "it all depends". Here are some thoughts.
Code actually used by other people is much more impressive than code done for a class or for learning (but those help too). So you might aim for something like web site development for a non-profit, or developing a free iPhone or Android application that people find useful, or contributing to an open source project.
Larger software systems are much more impressive than smaller ones, so if you can, take a college course that does multi-person long term projects instead of smaller machine problems.
Code you can show to prospective employers is more valuable than code you have to leave behind at a former employer. ideally, you'd like to be able to show source code as well as operation. If you aren't allowed to do this, you can always publish a blog showing how you solved specific technical problems, but using example code, not the actual code you wrote for your employer.
Tackling more complex problems is more impressive than working on simple problems, especially if you can do it with concise, elegant solutions.
Working closely with a few others on the project is also a good sign, assuming your contribution can be easily identified.
An employer is also going to be more impressed if your project(s) are in domains they find valuable, and if they use computer languages, frameworks, etc. that they need expertise in.