I’d also say to get up early for the sunrise at the top of Haleakala, and the bike ride down is a BLAST – a little scary when thinking about it, but so much fun.
Also, take a part of a day for driving the road to Hana, and make sure you stop every time you’d like to along the way.
There’s also great food in the town of Paia, along Baldwin Ave / Rt. 390. (Fresh Mint – # 115 – is seriously one of the best vege restaurants I have even been to, and well worth the wait).
Have fun!