Great question! First and foremost is your performance in your undergrad program. That shows how you handle the challenges of academia. Next is usually GRE/GMAT scores. Beyond that, some programs will look at subject related extra curricular activities. For example, if you are looking at a computer programming related degree, it would help to have a portfolio of open source and/or academic software contributions. It always helps to be published in a peer reviewed publication but that's quite rare for an undergrad so no pressure!
You might want to talk to the graduate advisor for the program you will apply to. They know better than anyone what really matters to that program.
Best of luck and I hope you get accepted to the program you want!