A free, practice LSAT is given by the Prelaw advisor every spring. Take it for an honest assessment of your probable performance. The combination of undergraduate GPA and LSAT scores form the overwhelming basis for most admission decisions on most applicants by most law schools. Among those already holding a bachelor's degree, obtaining a master's degree will not substitute for good LSAT scores. If you did not get into law school because of poor LSAT's, getting a master's will improve your chances only if your LSAT scores also improve.
Many law schools do confront an embarassment of riches in their applicant pools.
They receive many more academically attractive applications than they can accept. Among the pool of applicants with strong academic credentials, some marginal decisions may turn on letters of recommendation and extracurricular activities. Extracurricular activity and academic achievement are not commodities that can be evenly traded-off. Your extracurricular activity will do you little good unless your academic record places you in the competitive pool. Letters of recommendation are required.