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
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/
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
STEM at Home
https://smv.org/learn/stay-connected/stem-at-home/
CanTeach Science Resources
http://www.canteach.ca/resources/science/
Franklin Institute: Science Recipes
https://www.fi.edu/science-recipes
Utah Curriculum Resources: Elementary Science (K at bottom of page)
https://web.archive.org/web/20190209011508/https://www.uen.org/lessonplan/resource/core/1217
Paso Partners - Integrating Mathematics, Science and Language: An Instructional Program (K-3 units)
https://www.sedl.org/scimath/pasopartners/
Reach Out! Michigan Hands-On Science Lessons
http://www.reachoutmichigan.org/funexperiments/agesubject/subject.html
OLogy - American Museum of Natural History Kids Site (Click on a discipline and the hands-on tab on the page.)
https://www.amnh.org/explore/ology
Science Experiments and STEM Activities for Kids
https://littlebinsforlittlehands.com/science-experiments-and-activities/
Here are some sites that offer detailed lesson plans and guidelines for hands-on activities. They are grouped loosely by discipline.
Earth and Space Science
Teach the Earth
https://serc.carleton.edu/teachearth/index.html
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 K-4
https://www.nasa.gov/learning-resources/for-educators/for-educators-grades-k-4/
NASA Resources for Educators 5-8
https://www.nasa.gov/learning-resources/for-educators/for-educators-grades-5-8/
Physical Science
Inquiry in Action - Classroom Activities (NGSS-aligned lessons for K, 2, and 5)
http://www.inquiryinaction.org/
Adventures in Chemistry
https://www.acs.org/content/acs/en/education/whatischemistry/adventures-in-chemistry.html
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/
Engineering
The Concord Consortium
https://learn.concord.org/
TeachEngineering
https://www.teachengineering.org/
Try Engineering
https://tryengineering.org/
PBS Learning Media: Science
https://www.pbslearningmedia.org/subjects/science/
Science NetLinks Lesson Index
https://web.archive.org/web/20190716120353/http://sciencenetlinks.com/lessons/
National Museum of Natural History Teaching Resources
https://naturalhistory.si.edu/education/teaching-resources
California Academy of Sciences Lesson Plans
https://www.calacademy.org/educators/lesson-plans
In addition to lesson plans, you can also find many good activity ideas on the web. Here are some sites that offer detailed guidelines for hands-on activities. These are grouped loosely by discipline.
Physical Science
Jefferson Lab Teacher Resources - You'll not only find activity ideas but also videos, games, worksheets, puzzles, and more.
https://education.jlab.org/indexpages/teachers.html
Best of WonderScience Sample Activities
https://www.acs.org/content/acs/en/education/resources/k-8/thebestof.html
The Science Explorer
http://www.exploratorium.edu/science_explorer/
The following sites are from textbook publishers. However, you don't need to use the particular series to take advantage of the resources they have available online.
Houghton Mifflin Experience Science
https://web.archive.org/web/20071002073624/http://www.eduplace.com/science/hmxs/
Houghton Mifflin Science: Cricket Connections
https://web.archive.org/web/20130201145816/http://www.eduplace.com/kids/hmsc/content/cricket/