What are some activities for summer that will help me later in life?
Learning first aids,swimming,going to church.
Get your work permit and find a job, even if it's only a few hours a week. In the future it will reflect well on you with prospective employers when they see you took the initiative to begin working at a young age.
Start studyin for SATs they are a huge part of your resume (I'm 15 too) also try to get a job, something small that shows you hav experience working
Volunteer at an animal shelter! Get them to sign a paper saying.you did community service (which you will need for High school to graduate) and it looks good for colleges!