I live in Oahu. I recommend splitting time between Maui and Kona/Big Island. Humpback whale watching is the best in Maui, preferably February over March. I personally think whale watching is best from a raft since you're eye-level and you'll see more because the rafts are more maneuverable and can thus move faster toward whale action (I'd estimate seeing 10-20 whales per 90-120 minute tour in February which is peak season): Whale Watching Maui & Snorkeling Tours - Hawaii Ocean Rafting or Maui Whale Watching Tours and Snorkeling - Closest Encounter (HVG for 15% discount). Recommend diving Molokini (including backwall if you can), Lanai, and Mala Wharf while on Maui. Kona diving is the best diving in Hawaii in my opinion. If Kilauea is still erupting, I would recommend a day trip there.
I live on Oahu and couldn't agree more to what this member mentioned