An obvious starting point is to require portfolios to include one or more experimental investigations. The key here is to allow variety in the ways students can fulfill the experimental requirement while providing enough guidance and definition for the grade and skill levels of the students. For a biology portfolio, for example, various students may thrive in experiments involving live tadpoles, experiments measuring soil acidity in the classroom and experiments that require sampling water from several different outdoor locations.
Interview projects can help reinforce students' understanding of the applications and relevance of the research and experiments they conduct in science class. Talking with a professional scientist about her career, current research and personal history can help science come alive and connect to the real world outside the classroom in a student's perception. An alternative is to allow or assign fictional interviews with famous scientists in history. These can help students delve more deeply into the historical context and significance of important discoveries. Contact local scientists, researchers or professors to see if any are available to conduct interviews with students.
Students can enhance their understanding of the experiments they perform by backing them with research projects that investigate the scientific theories that explain the processes they are studying. Let each student choose a project that captures his interest while providing enough guidelines to make sure the research topics will be relevant to the portfolio theme and enhance the student's understanding. For example, in a "Force and Motion" portfolio, a student could research real-world results of the Coriolis effect, chart the forces that affect the motion of a race car or try to explain acceleration to students a few grades younger than himself.
You can help students make their portfolios more practical and beneficial by including at least one social assignment in the portfolio guidelines. This could include developing a plan or invention that uses the portfolio theme for a real-world benefit, teaching a useful concept to younger students (or family members or classmates), or exploring recent social trends or applications of the portfolio theme. For example, students making a botany portfolio could grow a roof garden or study the science that increases crop yield.