Strong students, particularly those interested in graduate school, should consider a summer REU (Research Experience for Undergraduates) or internship. Links to such opportunities, including off-campus programs like the Budapest Semester in Mathematics, are given below.
Eligibility and prerequisites for these programs vary, but generally you should have successfully completed the full calculus and linear algebra sequences at a minimum. Applications are typically due early in Winter quarter, and usually require a statement of interest and at least two faculty letters of reference. You should plan ahead and request these letters by the middle of Fall quarter.