If you're into snorkeling, I'd second Honolua Bay. The problem is that you'll need to rent stuff from somewhere else, because there are no gear rental places on sitr. Given your time constraints, stopping at a Maui Dive shop or something to pick up stuff might not be that efficient. A good second choice would be a K'anapali beach day. You can valet park for the day at the Sheraton and hang out near black rock. You can rent stuff for snorkeling right along the beach, and black rock has excellent snorkeling. You can walk to the outdoor mall -- Whaler's Village -- and get lunch, or eat at one of the hotels. (If on a budget, the deli/take out place at the Marriott is great.) You can stop at any of the activity places along K'anapali and sign up for a dinner cruise or parasailing or any of that stuff. If I had just one day in Maui, that would likely be my choice -- a K'annapali beach day. (Unless I was staying on K'anapali, perhaps.)
For the after 1:30 days, I'd probably do an evening in Lahaina. Warren and Annabelle's is a great choice, or just a nice dinner somewhere. Another good option for the after 1:30 days would be the Io Valley park. Nobody ever talks about it, but I found it to be tremendously underrated.