What she is already doing seems fine to me for most colleges. If she loves history, she could submit a project for National History Day. Winning at the local level would propel her to nationals and would be a feather in her cap. She could become a reading buddy at your local library or elementary school. She could also tutor elementary school students in math and/or reading. My children were tutors and found it immensely rewarding.
Joining a math circle is a popular activity for strong math students. Math circles are usually hosted by colleges. For example, there is the Stanford Math Circle. The math circle I am familiar with prepares kids for the American Mathematics Competition (AMC) and the American Invitational Mathematics Examination (AIME) and sponsors a team that competes in the American Regions Mathematics League (ARML). The kids who participate seem to really enjoy it.
There are myriad of ways to show well-roundedness. My philosophy has always been to follow my child’s lead and look for ways to expand their existing interests.