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I read excursion as "short vacation" sinceNote the OP said "quick" excursion. That would make Robert and Matt's suggestions the best options. Our dive park has plenty to offer (I know, I've dived it a few thousand times) and it is a pretty safe place for newly certified divers. However, Matt's suggestion of doing a boat dive from a mainland base like Long Beach or San Pedro would take you to two or three different Catalina dive destinations and be pretty cost effective.
At least from my perspective, I'd consider that divable time...has from the Friday before until day after Christmas off.