I'm doing something fairly similar shortly.
Arrive at Hilo at 10:40am.
Pick up rental car.
Day of driving and sightseeing.
(mainly have VNP/ Hilo / maybe the journey to Kona and back in mind)
Drop off rental car around 7pm.
Catch 8:40 plane back to HNL.
This thread has given me some good ideas! (Not too worried about the rushed day, I've had worse! If I love the place, I'll be back... actually already thinking of coming back to go to the astronomy areas.)