The Waterpark at Grand Wailea is a blast.
Any of the nearby properties make it an easy walk.

Many times, the Four Seasons next door has a really nice local art fare in the lobby. Fun to see the local arts on display.

The shops at Wailea is an easy walk from many properties. There are really nice shops, yes, but you will also find great restaurants, and nice coffee shops.
I like to hang out at Longhi's Restaurant/Bar. Free Wifi, and great place for breakfast/lunch/dinner without breaking the bank. Sit outside under the lanai!
Tell "Marko" I said HELLO!
There is also a Longhi's at Ala Moana on Oahu, so I frequent both of them a lot.

Also, Try Maui Bake Shop and Deli, 2092 Vineyard Street, Wailuku for a real breakfast or lunch treat. This place does not look like much from the outside (What does in Wailuku?), but the food is astounding. You have a genuine French Chef baking and making you food. Actually, most genuine I've had outside of Paris. And he does not compromise. His dishes are incredible. The baked goods are out of this world. If you have wheels and are anywhere near this place, stop by. Park anywhere along the street or side streets where you find room. Go in, and enjoy.

As a bonus, as long as you are on that side of the island, explore the shops and mall there. They are very nice. After all, you can't sit on a beach or play golf, or sit at a bar/restaurant the entire time, can you? Well, I guess you can! But stop and enjoy the local shops anyway. There are a bunch of them around that bakery/deli. Buy your wife something nice.

Hey, it's Maui, dude. You could have fun just BEING THERE! Snap a picture of yourself somewhere and send it to yourself and say, "Having a GREAT time, GLAD I am HERE!"