Just show her you have fun together.
Go out, do something fun; like bowling for example.
It's not too intense to call a date, but at the same time your both attracted to each other so you can see the sexual tension and attraction present during the time you spend together. Showing your a fun person, and showing her how to have fun generates memories, which can be looked back on in a positive way.
This other dude is more than likely going to take her for a drink or the cinema as a date which is B-O-R-I-N-G. And when she remembers how much fun you two had together, then it may just sway her decision.
Just make sure you don't turn all AFC on her and start buying her presents or sending "cute" messages telling her how you feel, and bombarding her with information that will end up pushing her away.
Stay cool, stay composed, and have fun.
Okay?
