If it's a weekend, the earlier you arrive, the better staging spot you get. It's a great place to take students and very popular with many divers. With that, it can get a bit crowded around the walls, but never unmanageable. Kind of sounds crazy, but once you get there, you'll see what I mean. Regardless, it's a rare day everyone's not in a great mood excited about the dives.
Take a mat!! You can pick one up at any of the local dive shops. It's nice to have something to stand on when you're putting your gear on, etc, since it's pavement. If the sun is beating down, it can get very warm on the feet.
If you want to dive the Valiant (it's outside the park boundary, but a lovely dive at 70'-90'), you need to get permission from the harbor master first. It's quite a swim out to the dive site but it's a nice dive. Inside the park you'll find so many wonderful things including the SuJac (Lynne mentioned above), glass bottom boat, sailboats, old swim platform and pipes, and a few memorial plaques. This on top of the kelp which is amazing by itself. There are a number of great websites that talk about the various features and you won't be at a loss for things to explore. Just remember to take your time and enjoy yourself. You'll be surprised at the life you might find hiding away (there's good chance to find an eel or two - I've rarely not found one around the old dive platform). I don't prefer diving one side over the other but will say that closer in the shallows, I've had great luck finding smaller macro subjects. Deeper water finds the huge kelp (bull kelp, I believe), which is definitely worth seeing.
Getting in and out is easy, but there is etiquette to follow (and I've ran into a few militant divers who don't take kindly when people do this wrong). Go down the right side to get in (as you're facing the water) and come up the left (as you're facing the water). Just watch a few other divers and you'll get the picture. If it's low tide, it can be a bit tricky with rocks, but otherwise, just wait for the right moment to hop in and crawl out.
Hope that helps and I hope that you two really enjoy yourselves!!