Lived in Kihei, mind you years ago.
South Maui has some great beaches. Sugar Beach is small, has a few shops. Was a great Mexican food place called Linda's. Down toward Wailea, good beaches, also. Ask about "Big Beach". It may have been developed over.
If you bar-b-que, grab some marinated meat from Azeka's Market in Kihei.
Do go to Kimo's for dinner in Lahaina.
Try some inland stuff, if there is time. The Iao Needle. From the airport in Kahului, look right and you should be able to see the Iao Valley about 20 minutes into the drive across the island (that's about half way)
The road to Hana is a must if you have an entire day to spare. That road used to go all the way along the south coast, but was not paved the whole way.
Haleakala is a sight, but again, time consuming.
You can head up to Makawao and at some points you will see both east and west coast from the same view. A little perspective.
Definitely the Molokini dive/snorkel.
There is surfing on the northeast shore, but I can't recall the name of the beach.
Enjoy!