Well, since no one else has replied yet, I'll "dive in" and see what I can do to help...
Unfortunately I live north of OC (in Ventura County) so I would not be the right person to tell you of the best dive sites in OC. But I will tell you that it's unlikely you'll find any dive-op/hotel combos. This area is very different from many of the destination dive areas you might visit -- there are no scuba-oriented resorts, and the dive shops around here generally don't offer dive/lodging packages. (Caveat: I could be wrong about this, there may very well be some hotel around Newport Beach that offers packages, but I've never heard of one.) Local dive ops do, however, often have boat charters and other scheduled dive trips, so that's something to look at.
Speaking of boat charters, that may be your best bet. We have fantastic local dive boats, and if you go on one of them for a couple of day trips, you will likely get the best diving experience you can get while you're here. There are a number of boats that go out from Long Beach (just north of OC) and San Diego. They will usually take you out to the Channel Islands, where you will generally find far better conditions than at the beach. Here is a link to a list of all of the dive boats in Southern California:
http://californiadiveboats.com. That site will give you links to all of the dive boats' web sites, where you can find their calendars and see if any of them have "open boats" during the dates you're here. You can also find out what organizations might have chartered them on your dates, and see if you can book it through them.
The reason I suggest going on a boat is because, with little ocean diving experience, and with your girlfriend being a new diver, it would not necessarily be a good idea to try beach diving. Depending on the surf conditions, entries and exits can be quite challenging -- even experienced divers around here are known to get "maytagged" on occasion, and there are few local beach divers who haven't sacrificed at least some gear to the sea gods due to heavy surf - something you probably don't want to deal with while on vacation. Further, unless you are diving with someone who knows the dive site, you may not know where to go to find the best reefs, and you could easily end up doing a dive in which you see nothing but sand, sand and sand. Boat dives will allow you to make easy, controlled entries and exits, and assure that you will get to see cool stuff.
However, if you are going to go on one of our local boats, you need to be aware of how they might differ from your other dive boat experiences. You mentioned you've been on a few warm-water dive trips -- this usually involves "resort-style" boat diving, in which the dives are led by a Dive Master, and your equipment is often handled with full concierge service. Our local boats operate differently: you are expected to deal with your own gear in terms of loading, unloading, set-up, etc. (although you can certainly expect a lot of help from our fabulous crews). Further, the DM's generally do NOT go in the water, but remain on the boat ready to hop in in the event of an emergency, and to help divers as they return. Divers are expected to be self-sufficient, are left alone to plan and execute their own dive plans, and must be able to get themselves safely back to the boat. This means that you need to ensure that you both feel ready and capable of diving on your own (and, of course, that's what being certified is supposed to prepare you for). It's just important to be aware of what to expect: you can expect to get a good, detailed dive briefing, but NOT to have a guide or DM-led dive. (You do always have the option of hiring a DM to be a personal guide -- a local dive op can help you with this, if you're interested.)
You can often rent your tanks directly from the dive boat; most local dive boats do offer air fills, and some even offer Nitrox -- check it out on the boats' websites.
Another option is to visit Catalina Island. You might even want to consider staying there for a couple of nights. Catalina Island is an easy 1-hour ferry ride from Dana Point (here's a link to the ferry:
http://www.catalinaexpress.com). Now, Catalina is definitely a more dive-oriented destination, and you may very well be able to find a dive/lodging package there - or just take the ferry there and back for a day or two. On Catalina, there is a dive park at Casino Point which has easy entries/exits and a wealth of things to see, as it is a protected marine reserve. You can rent tanks right there on the island, and get air fills for multiple dives from an air-fill trailer right at the dive park.
Well hopefully this gives you enough general information to start looking around and considering options. And I'm sure others who might be able to give you more information specific to diving in OC will weigh in.
