I am an incoming Sophomore. I have no internship lined up, no summer program registered for.
Summer school doesn’t seem worth it at this point.
Interests include social studies/law/economics/humanities.
Ideas for how I can avoid letting my first summer of high school go to waste?
Volunteer? There’s bound to be plenty of places to make a difference in your community.
Summer reading 
Just kidding, I don’t know if you have to or nah…
Volunteer. Start a new hobby or ten. Check out the SAT/ACT and familiarize yourself with them. Search for competitions/contests you could enter to possibly win awards. (Try to) learn a new language. Run every day. Get a head start on your hardest classes next year. Think of a couple of research ideas and email some professors.
^ ^ Great suggestions!!!
Google “volunteer opportunities for teens in ____.”
With a little work you should be able to come up with quite a lot.
Concurrent enrollment in a local community college, but it’s probably too late to sign up 
You should definitely do this next year! I’m taking Sign Language this year. Sign ups are usually at the very end of May for high school students.
Volunteer, volunteer, volunteer!! It’s such a fun and amazing experience. I use volunteermatch.com
Are you taking AP classes next year? If you do then you probably have summer homework.
CAMPUS TOURS! They are so much fun and easy to do. They’re free and you only have to pay for traveling. This summer I’ll be vising UW, UCSD, UCSB, and UCLA again 
Online classes- there’s a site (sorry, I forgot it!) that provides online classes by top universities like Stanford that actually gives you certification. I tried one class but never continued just because I can’t study online, I have to be in a classroom.
Sports. If you already don’t do a sport, you should definitely go to open gyms with friends. Your schools should have them and if not, join clubs/leagues in your town. For summer, tennis, golf, and volleyball are the most popular because they’re fall/summer sports.
Scholarships. (I use fastweb.com) It’s never too early to start saving money for college. Some of the topics are really fun to write about, too.
Shadowing- I don’t know what types of jobs you can shadow that are in your interests but they’re available. For example, at John Muir Hospital, you can shadow a doctor for 4 hours. This helps you see what you want to do in life and if you’ll be happy doing it.
Friends? Don’t lock yourself up during summer! You have so much time to build stronger relationships and make amazing memories with your friends OR meet new ones.
Have a great summer!
Learn a language! Use doulingo, its a free language learning app. Something I did was I went to a used bookstore and ended up finding textbooks for very cheap, try that and teach yourself something!
It never hurts to seek community service hours. You could go get fit at a gym if you look forward to extracurricular so. Oh, you can go for college tours with this summer. You want to get a good feel of the schools you plan to apply for in the future. That’s what I would do, I don’t find it too early at all to be taking a college visit, even though they probably get you to do it in junior year.