Ways to demonstrate ("show-off") contributions as an undergraduate in research I am an undergraduate   CS  student at a US college. Before this summer, I contacted a professor in Game Research and was offered the opportunity to work with one of his PhD students. Currently, I am focusing solely on this opportunity. I wrote lots of code and helped the PhD student run experiments and collect data.

The best outcome of this opportunity would be a publication and a recommendation letter. However, I am only 50% confident that I will be able to get one of the two at the end. 

Are there other practical ways for me to demonstrate my contribution (that can help with future applications)?

Any other advice would help too.
A recommendation letter is the way you get credit for this kind of work. I'm not sure why you think you're only 50% likely to get a letter out of this, it would be very unusual for a professor to be willing to work on a research project with you but to not be willing to write a letter for you.  As long as you're taking it seriously and professionally I think you should have close to a 100% chance of getting a letter out of it.