There are lots of sources for ready-made lesson plans that you can adapt to suit your own purposes, depending on your students and their individual learning outcomes.
I have collected my best sources of science lesson plans:
The Special Education Support Service has a great range of resources including lesson plans, experiments, worksheets and a toolbox for supporting students with special needs in the science classroom. You can download the entire pdf kit for free (3MB). This is one of the resources I have provided in the Digital Toolbox on USB.
science_class_net.jpg is another excellent site for traditional lesson plans from a very experienced science teacher. I especially like his scientific method with paper airplanes lesson.
The Science Spot also has a range of lesson plans and other resources from an experienced science teacher. Check out the links to Mythbusters, Consumer's Challenge and Old Wives Tales in the "General Science Lesson Plans" page.
Middle School Science Lesson Plans has a search engine for finding science lesson plans, ideas and activities for middle years students.
Teachnology has a range of lesson plans contributed by teachers. "Moon Craters" and "Using a Ramp" are two fun activities that give students the opportunity to understand how meteorites make craters and how the angle of a ramp affects the distance a vehicle travels.
Discovery Education has a teacher's section with free resources, such as clip art, lesson plans and science fairs.