You didn't say how experienced you are, or what you're interested in. Personally, I think it's hard to beat the backside of Molokini, but getting to dive there depends on conditions, the dive operator and experience level of everyone else on the boat. And the second dive will be somewhere close to shore.
After diving Maui for many years, I just found out about the hammerhead dives off of Molokai.
Where you stay will also affect how far you're willing to drive to get to a dive boat. If you're staying way up by Kapalua, it's a long drive in the morning down to the Kihei launch ramp, or if you're down in Wailea, it's a long drive to get up north to go out to Lanai
You can also probably find someone local to do shore dives (early, late, night), so you might be able to sneak in more than just one day of diving
Enjoy Maui!