I've been to Maui 3-4 times, most recently on my honeymoon in fall of 2014. Definitely my favorite island (although I became more partial to the Big Island after visiting for the first time in a long time in 2013). Honolulu is not bad, but to be frank Oahu is the island I have visited the least, in large part because it is so much more populated / crowded than the others. It's definitely worth going to at some point, but I would make Maui your first stop since this is your first trip to the islands. Flights - others can probably recommend better if you are flying out of SAN.

In terms of what to do there: Road to Hana is a must since it's your first time. This is a LONG drive (don't get confused by the short distance - it is a ton of curves and winding), and given how popular it is, I would definitely head out on the earlier side. I think OP mentioned above, but if it's your first time, get the CD that goes along with the drive - very informative, points out good places to pull over and do some sightseeing and walking. If you head out early, I would also go to the Seven Sacred Pools, which is just beyond Hana. AND if you are feeling adventurous / rent a car that can handle a little rough road, I would drive along the backside (i.e. southeastern) side of the island. Extremely peaceful and serene, with very few cars or traffic as well. If you're rolling the dice on a rental car that doesn't technically allow offroading, it's really only the first ~10 minutes past the pools that can be a bit challenging, but the rest is not a problem.

Haleakala, as others have mentioned, is a must as well. A lot of switchbacks while you drive up the mountain, but it's not too bad, and you park right at the crater. How much you want to hike into the volcano is really a matter of how long you want to be out there. I would also recommend going up for either sunrise or sunset as well if you aren't looking for a long hike - I've been up there for sunset (it was a day when the summit was through the clouds), and it was a gorgeous sight to behold.

If you're looking for a very scenic - and mildly challenging (especially if it's raining / misty, as it was when we did it) hike, I would recommend the Waihee Ridge trail. You get great views of central Maui and the northern coastline as you ascend, and you have a great view of waterfalls and the valley (I am told - again, hiked this on a day with really poor visibility) at the end of the trail. Nakalele Blowhole is also a bit of a challenge to get to if you decide to make it a rock scramble, but that's also cool to see on the northern edge of the island.

Iao Valley is worth visiting as well - the needle is a great natural spectacle to look at - but that won't take as much of your time. If you are into snorkeling, definitely make the trip out to Molokini...I went with Trilogy, and while I would pass on the super-sweet cinnamon rolls before getting in the water, they do a great job, and you get to see tons of fish. We saw quite a few dolphins in the water as well. Upcountry is worth a visit, if to see a bit more of the rustic side of Maui. If you are into farm-to-table at all, O'o Farms is a fantastic mid-day activity - pick some of your own vegetables and enjoy a fresh meal made at an outdoors kitchen. Your views (facing west) will be very expansive of the rest of the island. The winery listed above is towards the end of upcountry (where you would actually loop around the back end coming from Hana). I wouldn't make a trip just to go out there; to be frank, the wine is not anything special, and their pineapple wine (which is the main unique thing about them) isn't all that great, IMO.

With regards to accommodation, I have NEVER stayed in Lahaina / Ka'anapali when I am on Maui, with the main reason that it feels much more jam-packed and touristy than the Kihei / Wailea region of Maui. I don't mind visiting Lahaina when I go to visit - checking out the crafts market under the banyan tree is a must, and if it's your first time, I would also recommend seeing Ulalena as well (if it is still there), but Kihei and Wailea are much more relaxing, chill parts of the island to stay at. While it's not that far anyways, staying in western Maui will add a little bit of time on to your trips, as most of the scenic stuff (Haleakala, Hana) are on the eastern side of the island. I stayed at the 4S in Wailea last time I was there, but the Andaz and the Fairmont (which are on the same coast and adjacent) are also very good properties (with the Andaz having the best dining, IMO, with Ka'ana Kitchen). They are all west-facing, so you will have great sunset views (same with any at Lahaina / Ka'anapali as well).

As for a car, we rented a convertible last time from one of the local places. While the condition wasn't great, there is nothing like cruising around Maui with the rooftop down. Do it!