I only get to Southern California every month or two, so I also find myself looking at day trips to Catalina when the conditions in Laguna aren't ideal. But if the swell is down and there's no red tide, Laguna has some terrific diving, which you shouldn't underestimate. Shaw's cove is a good place to start.
To decide if you need to go to Catalina or not, first check the swell models
http://socdc.brinkster.net/Ocean.html and the current conditions
http://www.lagunaseasports.com/conditions/conditions.asp. You probably want swells under three feet and visibility of at least 10-12 feet. (Don't expect to ever have viz more than 25 feet or so on the mainland, which is probably the minimum viz at Catalina.)
If you want to do a shore dive in Laguna, you can join up any weekend morning with this group:
http://southcoastdivers.com/, and they will make you feel at home.
Otherwise, I like the Island Time/Express Divers. They have half-day two-tank dives to Catalina
http://www.expressdivers.com/ that get you back around lunch time. The other divers are often in classes, so check with the boat if there are other single divers, or try to bring your own buddy. I don't really like taking the Catalina Express to the dive park unless I'm planning to stay a while in Catalina. It's a bit of a hassle getting your stuff to the dive park and back (although the dive park is a lot of fun). If you take the Island Time or another dive boat, you don't have to worry about schlepping your gear, watching your gear, buying lunch, paying for parking at the ferry, etc.