All of the above are great suggestions.
Just on the north side of Lahaina is Baby Beach. I highly recommend grabbing a 6 pack or two of Maui Bikini Blonde and hanging out there for a few hours in the afternoon. You can watch the sunset from there. Enjoy a nice beer sitting in the thigh deep water. It is perfect because the reef goes out 100 yards and the waves break way out there.
If you are a diver, the Maui Dive shops offer great day trips on a boat, or you can do beach dives. If just snorkeling, it's cheaper, but still fun to go off a boat. If you're staying from the shore, Honolua Bay is great for that.
If you do drive to Hana, keep going and hang out on Hamoa Beach. It's a little gem. I also recommend that you keep driving around the island, even if your rental car contract says not too. Just be careful of the cows. If you are driving back at night around the long way, make sure to stop occasionally and enjoy the stars - there is no light pollution!
Makena Beach is also really nice, and I suggest hanging out in Wailea, too! You can't go wrong by walking on the path between the various hotels. The Marriott there is beautiful, and when the lounge chairs aren't occupied or reserved, the staff usually lets you stay on them (without the cushions) but still enjoyable. Find an empty hammock and you're set.
I also recommend Palauea Beach. It's a lot more private and not very touristy.
Eats:
Kihei Cafe - great great GREAT breakfast!
818 Deli has great sandwiches.
Maui Brewing Company Brewpub
Have fun! I'll be there for 5 days in mid June staying in Kihei.
https://blog.alaskaair.com/destinations/5-best-maui/