The Exploratorium's Teacher Institute calls small science activiites "Snacks". These activities are tried out with students and teachers and have all the information you need to make them work. The instructions are 100% free, and we don't sell parts or kits. If you need help finding something, well, that's part of my job.
These days I work for the Exploratorium, so much of my work now ends up here. The work doesn't have my name on it, since we work collaboratively, but some I helped bring to the page.
Some of my favorites include