Visit the southern point of the USA. You never know when you'll want to visit the geographic extremes of the USA.
Visit the winery along the road north of the south point; you never know when you'll want to visit a winery in every state.
Try to visit a rainbow of beaches on the Big Island; we made it to a white sand beach, yellow sand, green sand (near south point), and black sand (nude beach south of Hilo).
Mauna Kea is a cinder peak and Mauna Loa has a hard lava surface. To climb it, go to the saddle and turn south (north is Kea). Loa is also the highest pont of the national park; you never know when you'll want to visit the HP of all 58 national parks.