A good science fair project at any grade level should include a hypothesis, background research, an investigation, data collection and a presentation of the results. An example of a successful fifth grade science project is exploring skyscraper design with Lego.
A successful science project has a clear hypothesis that can serve as the focus of experimentation. The Lego project starts with the fifth grader's design idea of what makes a tall building able to withstand environmental forces. The design of the skyscraper probably comes while the student is conducting research.
The next steps for this project include building the skyscraper out of Lego and testing it via a simple shake plate. Fifth graders can also devise ways of testing the construction for wind and water forces. At every step of the way, they need to keep concise data of what they're doing to the building and what the results are. This data serves as the basis for their reports.
Typically students display the results of their project on a display board. This is a trifold poster board with specific elements. The board should include the initial hypothesis and background research on one panel. The center panel displays the materials, procedure and results. The final panel shows the conclusion and either graphs or pictures.