I have my 7th grade science students keep a binder for my class. Each semester, they go through their binder and pick 5 "artifacts" and put them together in a very simple portfolio. They make a cover page, include their artifacts, and explain (on a guide sheet that I created) why they picked to showcase those particular artifacts. After putting their portfolio together, they clean out their binder and start over for the next semester.
The document is in Word, so feel free to modify as needed for your class. I included the simple rubric that I use at the end of the document. This would work well in any class- not just science!