How can an undergrad absolutely impress Research Professor during a research internship?
The solution is straight-forward.


Gain enough knowledge of what your research group is progressing on
Develop skill-sets that would aid to to work in the group


Better yet, contact the professor and ask if there is anything you could do get prepared for the research internship. This would not only portray your enthusiasm and dedication, it would give you a clearer idea on what exactly to improve.