I spend most summers camping, traveling and working on my own research projects (and sometimes all three at once.) This summer -2022- I plan to serve as a deckhand/educator on the tall ship Matthew Turner. But otherwise I might be tempted by these:
The National Park Service Teacher Ranger Teacher Program places teachers in one of over fifty participating national parks to learn about the park and develop curriculum. My own local park, the Point Reyes National Seashore, is one. (Point Reyes was even hiring in 2020 when everything else was shut down.) You get a stipend, graduate credit, and sometimes on-site housing. https://teacherrangerteacher.org/
The Ignited Summer Fellowship Program places San Francisco Bay Area teachers into summer fellowships with Bay Area technology companies. There is a stipend of $7000 – $9000. http://www.igniteducation.org/teachers/become-a-fellow/
The Cornell Lab of Ornithology’s Summer Educator Retreat. You pay ~$500 but it gets you four days of bird science at Cornell (Ithaca, New York), meals, binoculars and other materials and credits from Cornell. But, you also have to pay your own transportation and lodging. http://www.birdsleuth.org/summer-retreat/
San Francisco’s Exploratorium has a great Summer Institute Institute for teachers. Three weeks; you get paid a $2,500 stipend at the end of the program.
https://www.exploratorium.edu/education/teacher-institute/summer
Lawrence Berkeley National Laboratory’s “Physics in and Through Cosmology Workshop” is free and involves four days of particle physics and cosmology on UC Berkeley’s campus. Advanced high school students are eligible to attend too; one of my students did. https://sites.google.com/lbl.gov/quarknet-workshop/home
The McDonald Observatory Teacher Workshops are held at the University of Texas’s observatory in West Texas. $100 gets you food, lodging, programming (deep space stuff!) and observing. https://mcdonaldobservatory.org/teachers/profdev
Teacher Workshops in Conservation Science at the San Diego Zoo: lodging and meals in San Diego are paid for by the program, and you even get a $500 stipend. https://institute.sandiegozoo.org/teacherworkshops
Stanford University’s Office of Science Outreach has a number of programs on campus for teachers, including paid summer research fellowships. http://oso.stanford.edu/
EinsteinPlus workshop for teachers at the Perimeter Institute for Theoretical Physics: A $300 fee gets you a week of lodging, meals, and theoretical physics in Waterloo, Ontario (near Toronto.) Canadian teachers even get airfare. Teachers from other countries need to buy their own plane ticket. http://www.perimeterinstitute.ca/outreach/teachers/programs-and-opportunities/einsteinplus
The Coastal Marine Biolabs teacher training on the Barcode of Life Program pays a stipend, and it will probably transform your teaching of molecular biology.
In Fermilab’s Teacher Research Associates (TRAC) Program, teachers are paid $1000/week! Fermilab is an experimental physics facility near Chicago. https://internships.fnal.gov/teacher-research-associates-trac/
Cal Poly’s STAR Teacher Researcher Program can set you up with a paid research fellowship at a number of California labs such as those run by NASA and the Department of Energy. There is a $5000 – $6000 stipend. This program is for current California State University students and recent graduates of the California State University system.
Frontiers in Physiology is the American Physiological Society’s (APS) flagship program for the professional development of middle and high school science teachers.
The Montana Natural History Center’s “Wings Over Water” Program is a week of ecosystem science, featuring Ospreys. Everything is paid for, except for getting to Montana. https://www.montananaturalist.org/wings-over-water/
The Monterey Bay Aquarium Research Institute has a summer internship program with a stipend.
The Denver Museum of Nature and Science’s “Foothills to Tundra” course sells out quickly – and no wonder. $300 gets you four days of food, lodging, and day trips in Rocky Mountain National Park. https://www.dmns.org/learn/teacher-professional-development/k-12-professional-development/
Chabot Space & Science Center offers teacher workshops.
NASA’s Texas Space Grant Consortium offers weeklong professional development training for teachers. This aerospace workshop, called LiftOff, emphasizes science, technology, engineering, and mathematics (STEM) learning experiences by incorporating a space science theme supported by NASA missions.
National Geographic Network of Alliances for Geographic Education offers world-class professional development opportunities throughout the U.S.
Plus, a lot of the free teacher travel opportunities l have listed are in the summer – such as California’s Forestry Institute for Teachers: http://www.forestryinstitute.org/.
These are organized programs that you apply for. 
It’s also possible to build your own customized summer opportunity – which I did in 2018, spending several weeks at the University of California’s White Mountain Research Center on a self-designed archaeology/history project. My book discusses real-world examples of how teachers have started a project, obtained affiliations, collaborated, applied for things, traveled with purpose, taught what they learned, and published the results. So invest $15 and support a fellow hardworking teacher!