NIRSEF participants compete to advance to the state level, HSEF, where they compete against other students around the state to go to the national level, ISEF. In addition to the Intel International Science and Engineering Fair, there are some additional competitions you may want to enter as well.
Regeneron Science Talent Search is for students currently in 11th grade who have completed individual research projects (teams not allowed). More information can be found on their website.
I-SWEEP: The International Sustainable World (Engineering Energy Environment) Project olympiad draws hundreds of the brightest STEM-focused students in the world together each year to compete in a fun and collaborative environment. More information can be found on their website.
Genius Olympiad: GENIUS Olympiad is an international high school project competition about environmental issues. It is founded and organized by the Terra Science and Education and hosted by the State University of New York at Oswego. GENIUS Olympiad will host projects in five general disciplines with an environmental focus. More information can be found on their website.