Answer: How much free time do you have? If you have the time and can afford it, I'd recommend traveling to a language school somewhere. You can not only learn Spanish, but also see another culture, make friends and see interesting sites. Most schools will give you an exam at the beginning of your studies, so you can get a class — or at some schools, individual instruction — that's geared to your level.
Except for the air fare, it can be done quite cheaply in some places, such as Guatemala or El Salvador, where you can get a room in someone's home, meals and instruction for under $200 per week (and sometimes well under that, although quality of instruction and food can vary). Schools in countries that have a higher standard of living, such as Spain and Argentina, cost more, although costs are usually still competitive with other types of travel. Although the more time you have the better, in my opinion even a week or two is worthwhile, especially if you're in a situation where you can spend a lot of time in conversation. And while you're there, you can buy materials such as Spanish-only books and Spanish-language music that may be difficult to find at home.
If you can't leave your hometown, look for a place where you will be able to have conversation with Spanish-speaking people. Some churches have programs for Spanish speakers, or you may be able to find a Hispanic organization and/or contribute time to an agency or group that works with Spanish speakers.
There are academically more rigorous places for working on Spanish during the summer, including distance-learning programs at some colleges. But unless you really want to spend your summer doing what you do during the school year, I'd strongly recommend finding some way to actually use the Spanish you know rather than formally studying it more. There'll be plenty of time for formal study once school resumes in the fall.
