Hi, My daughter is in second grade and likewise needs to come up with a science project soon. When she was in kindergarten, we discovered science fair project ideas on Education.com, and chose one that won her a first place ribbon in the fair. So this is going to be the first place we look for her second grade project as well!
It's important that you and your child pick a project that will keep her interest, and that you review guidelines that the teacher has provided (for instance, may the student do a demonstration project, or must the project be an experiment?).