Wow. You're a college student, and so you have lots of skills. Why not use them during the summer? Better yet, why not do something that will also help you determine what you want to do later in life?
If you want to know what research is like, try doing research in a Research Experience for Undergraduates (REU) program. If you're thinking of graduate school, trying an REU is almost a must! They're lots of fun, and you get paid, too. Application deadlines are mainly in February and March.
REUs receive many many more applications than they have spots, so it's a good idea to apply to at least 6 of them. They also vary widely in structure (for example, whether students work individually or in groups, on similar projects or different projects, with lots of faculty supervision/interaction or very little). If you've been accepted to more than one REU, be sure to ask about the details of the structure if these aspects of your working environment are important to you.
Many programs are restricted to those undergraduate students who will not have graduated before the program begins (that is, they exclude current seniors). The following five programs do accept applications from graduating seniors:
The Center for Discrete Mathematics and Theoretical Computer Science (DIMACS) has three REUs.
The Park City Mathematics Institute (PCMI) has an Undergraduate Summer School which interacts with the Graduate Summer School and Research in Mathematics programs.
NIST SURF is the National Institute of Standards and Technology's Summer Undergraduate Research Fellowship with projects in applied mathematics. Applications are made through your undergraduate institution; the person to contact to start the process may be listed here.
IPAM is the Institute for Pure and Applied Mathematics. It hosts the Research in Industrial Projects (RIPS) for undergraduates and recent graduates.
The National Security Agency has a variety of summer programs, some of which are research-based (such as the very-exclusive Director's Summer Program). They all have October 15th deadlines.
Just for Women:
The Institute for Advanced Study (IAS) hosts an 11-day workshop
for Women and Mathematics
each May.
George Washington University hosts the Summer
Program for Women in Mathematics.
Carleton College hosts the Summer
Mathematics Program for Women Undergraduates. It's for rising sophomores
or juniors.
About to go to graduate school? Consider the EDGE
program for the summer before you start.
If you're interested in trying out the world of industry (business, actuarial, technology, etc.) you might look for an internship.
What if you want some teaching experience? Try looking at middle-school and high-school enrichment programs---many of them hire undergrads to assist with teaching or homework.