I’m a high school junior, and I really wanted to do something impressive this summer to boost my college application.
So, I applied to some summer programs: SIMR, SSP, COSMOS, Girls Who Code. I’ve been rejected by the first three; waiting on the last one, but three rejections in a row isn’t helping my confidence.
What can I do over the summer to boost my application? Are there any good summer programs/classes that I could still apply for now? I could get a job but I was hoping to do something academic…
Good luck on Girls Who Code. Don’t worry, you’ll probably get in. 
You could check with your local community college and take classes there; many colleges accept credit for DE classes, especially in state (for example, the UCs and CSUs accept credit from my local cc). My cc has courses from Art History to Chemistry offered during the summer sessions along with online and fast courses.
In addition, I know some of the UCs have summer sessions, like UC Berkeley and UCLA, so check them out. I’m not sure if registration is still open.
Anyway, I wish you the best of luck on Girls Who Code! Three rejections doesn’t mean you won’t get into GWC, so don’t be discouraged.