Summer is the best time for any student because you get a break from school and time to yourself to do whatever it is you want. In college, summer is a time to build off of what you have been doing in school. Summer should be a productive time where you apply yourself to things you can not during the school year. Early in the year is the time to start planning things to get into. You can get involved in summer classes/study abroad, volunteer work, internships, and even a job.
The best advice I can give anyone about taking summer classes is to take them at a local community college. This can save a lot of money since FAFSA does not provide aid for summer classes. If you are interested in doing this though, you must talk to your counselor to see which classes can transfer over and which classes you simply can not take out of your university. Courses that are considered core courses or ones that are not taught in a universal way (ex.: calculus) should not be taken outside of your university. However, it is always good to take one or two summer courses to stay in the swing of things and get ahead on some credits to take the load off in regular semesters. As for study abroad, I know that this provides people with a lot of great opportunities and experiences that are unique. I personally have never done one yet, but everyone I know who has absolutely loved it. Each school has numerous resources for those who want to participate and you can learn more through your counselor.
With lots of more free time on your hands you can get involved in some local volunteer opportunities that you can find. It is pretty easy to find organizations to devote your time to by looking some things up. Your university probably has a department or website dedicated to finding students opportunities to engage with their communities. The vast majority of post-graduate routes you will take either requires or strongly suggests some amount of hours of volunteer work, which is not a commonly known thing. Admissions committees in particular love to ask about any volunteer work that you did and what it meant to you or how it shaped some part of you. Whatever volunteer work you do should should have meaning to you, so much to where you feel connected to it and not like you are just checking off a box.
After we all earn our degrees, most of us will get a job and work until retirement, so work and internships might not sound the most appealing. However, these two activities are probably more important than grades. These get you exposed to what is to come and prepare you. Your internships should be tailored to what you want to do in the future so that you can a) see if this is really what you want to do almost everyday of your life and b) get your foot in the door–make connections, apply your classroom knowledge, be emerged in the environment, etc. Any post graduate route loves to see internships on resumes and applications. They like to see that you have some familiarity and want. If you get a job, then this does not have to be geared towards your path and post-grads still like to see that you worked, even if you had a waitressing job but want to be a nurse. It is good to show that you picked up skills along the way that school could not teach and you have those skills to bring to the table.
All of these engagements build your character and let you find more out about yourself, even summer school. These things elevate you for your future, so plan for them early on, find opportunities and put yourself out there for them, and get something out of them. You are still planning for summer though so make sure to reserve some poolside days too!