What should a physics major spend their summer doing? More physics of course! But this doesn't mean more time in the classroom. The summer is an ideal time to learn the actual practice of physics by working on a research project. Below is a (non-exhaustive) list of options and opportunities. It is only intended to be a starting point for some of the more popular programs; you should search far and wide for others! Just Google "*name of research institution* internships" to get started. For example, interested in biomedical research? A quick search leads to the National Institutes of Health. Also look for companies in your area of interest as well.
Research Experience for Undergraduates (REUs)
The National Science Foundation supports numerous REU programs at universities around the country. These programs cover a wide array of fields, including Astronomy and Physics, but also Chemistry, Computer Science, Biology, and more! The program will pay housing, travel, and a stipend for ~10 weeks at the hosting institution. Applications are typically due around February to March. Note that you have to be a US citizen or permanent resident to apply.
There are various ways to get into a national lab for the summer (and spring and fall too!), the primary one being Science Undergraduate Laboratory Internships (SULI). Many, but not all, national labs participate. The ten-week internship (sixteen weeks in spring/fall) pays $650/week plus travel. Applications are due 4-5 months in advance (early January for the summer one). As with most government programs, US citizenship or permanent residency is required
Some labs, including Lawrence Livermore, Los Alamos, Fermi/Argonne, and Sandia offer separate programs.
You can also find a number of opportunities through the Oak Ridge Associated Universities. One opportunity advertised there is the DOE Scholars program (applications due in December!).
NASA offers tons of summer internships, all searchable from their website. With one application, you can you apply to up to 15 opportunities. Note that US citizenship is required.
Pathways to Science
Managed by the Institute for Broadening Participation (IBP), this site has a searchable index of over 650 paid summer research and internship programs in the STEM fields (note that many will overlap with the other resources listed above).
SPS, AAAS, and APS
The Society of Physics Students offers an internship specifically focused on science policy (due in January); they also track other job opportunities. The American Association for the Advancement of Science offers policy internships as well. The American Physical Society lists a number of external internships. While many are overlapping, a few are not.
Joining a lab at Georgia Tech
When in doubt, you may be able to find a lab here looking for students in the summer (and beyond!). Do not limit your search to the Physics department, but also consider the College of Engineering as well.