If you want students in your class to hear from scientists about their work, check out this site.
Skype a Scientist
https://www.skypeascientist.com/
For a focus on the nature of science and how scientists do their work, check out this site.
Understanding Science
https://undsci.berkeley.edu/
There is a wealth of high-quality, ready-made science lessons on the web. Here are sites that encompass a wide range of science topics.
PBS Learning Media: Science
https://vpm.pbslearningmedia.org/subjects/science/
National Museum of Natural History Teaching Resources
https://naturalhistory.si.edu/education/teaching-resources
Science in the City Resources
https://scienceinthecity.stanford.edu/resources/
Exploratorium: Digital Teaching Boxes
https://www.exploratorium.edu/education/teacher-institute/digital-teaching-boxes
California Academy of Sciences Lesson Plans
https://www.calacademy.org/educators/lesson-plans
HowtoSMILE.org
https://www.howtosmile.org/
National Science Foundation Classroom Resources
https://www.nsf.gov/news/classroom/
NOVA Labs
https://www.pbs.org/wgbh/nova/labs/
CK-12 Teacher
https://www.ck12.org/teacher/
National Park Service Educator Resources - Search by subject and grade level
https://www.nps.gov/teachers/index.htm
Smithsonian Educator Resources: Science and Nature
https://www.si.edu/educators/science-resources
Here are some sites that offer detailed lesson plans and guidelines for hands-on activities. They are grouped loosely by discipline.
Life Science
BioEd Online Lessons and More
https://www.bioedonline.org/lessons-and-more/
BioInteractive
https://www.biointeractive.org/
Teach Genetics
https://teach.genetics.utah.edu/
The Biology Corner
https://www.biologycorner.com/
Understanding Evolution
https://evolution.berkeley.edu/
Earth and Space Science
Infiniscope
https://infiniscope.org/
Earth Exploration Toolbook
https://serc.carleton.edu/eet/index.html
Teaching Climate
https://www.climate.gov/teaching
Teach the Earth
https://serc.carleton.edu/teachearth/index.html
Kinesthetic Astronomy
https://www.spacescience.org/eduresources/kinesthetic.php
USGS Educational Resources
https://www.usgs.gov/educational-resources
NOAA Ocean Exploration Education Resources
https://oceanexplorer.noaa.gov/edu/welcome.html
NOAA Resources for Educators
https://oceanservice.noaa.gov/education/for-educators.html
NASA Resources for Educators 5-8
https://www.nasa.gov/learning-resources/for-educators/for-educators-grades-5-8/
NASA Resources for Educators 9-12
https://www.nasa.gov/learning-resources/for-educators/for-educators-grades-9-12/
Physical Science
Inquiry in Action - Grade 5 Lessons
https://www.acs.org/education/resources/k-8/inquiryinaction/fifth-grade.html
Middle School Chemistry (NGSS-aligned lessons)
https://www.acs.org/middleschoolchemistry.html
Celebrating Chemistry
https://www.acs.org/education/outreach/celebrating-chemistry-editions.html
Periodic Videos
http://www.periodicvideos.com/
Jefferson Lab Teacher Resources - You'll find not only activity ideas but also videos, games, worksheets, puzzles, and more.
https://education.jlab.org/indexpages/teachers.html
The Science Explorer
http://www.exploratorium.edu/science_explorer/
PhET: Interactive Simulations
https://phet.colorado.edu/
Universe & More: Interactive Physics Resources
https://universeandmore.com/
Engineering
The Concord Consortium
https://learn.concord.org/
TeachEngineering
https://www.teachengineering.org/
Try Engineering
https://tryengineering.org/