I did a dive to 1st Cathedral at Lanai with Extended Horizons from the Mala ramp in Lahaina. Relatively short boat ride from there and no more than 6 divers per guide. Since the sea was a little rough that day, and some folks on our boat were sea sick, they came back in to do the second dive at Mala, which is a nice shore dive location too. Mala is a wrecked pier in Lahaina not far from the Cannery Mall. It was convenient from the boat but also can be done from the shore.
Extended Horizons web site is
http://www.extendedhorizons.com . They have some nice t-shirts which they will sell you after the dive if you are interested..
I also did a dive to Molokini with Maui Diamond II from Maalea, which is slightly closer than Kihei if you are coming from Lahaina, but really it's 6 of 1 and 1/2 dozen of the other as to whether to drive a little farther to Kihei boat ramp or take a slightly longer boat ride from Maalea. That boat really tends to pamper you, but they also cater to a wide variety of divers - they may have a different instructor with intro divers, and they would have a divemaster with certified divers. They serve some nice snacks and sandwiches and also throw out some fishing lines on the way out and back. The owners dog rides on the boat and actually gets in the water and will swim or ride on the back of one of the divemasters while she swims around. These folks were really nice.
As for shore dives - Airport is a nice easy sandy entry - free parking there - go north on 30 and turn left onto Kai Ala drive, then bear right. The park is called something other than airport beach, but you'll find it. You're near Black Rock, which is also a nice easy entry.
Maui Dive Shop has a store at Whalers Village Shops so you can get tanks from them.
I would recommend a shore dive from Honolua Bay, up north on 30 past Kapalua - keep going, road narrows and you will see first Slaughterhouse Bay - some steps going down there - I haven't dived that one, but other folks say it is nice too. Honolua is the next area. You will see cars parked along the road. Parking is hard to find there and don't leave anything valuable in your rental car. It's a bit of a hike down to the entry, especially with gear on. It's a rocky entry but not hard if you are careful.
Down in the south - Wailea area you have Ulua Beach which has a nice reef and easy entry, but when I was there the south was getting a bit of surf and vis was not very good. Probably not worth the drive from Lahaina considering that.
I would definitely try a night dive - contact Extended Horizons or
Maui Diving and they can provide guides for you along with all equipment for the night dives.
Maui Diving will also provide a guide/instructor for shore dives if you like and I think they are cheaper than some other places, but you will want to shop around.
Enjoy your trip.