The pier is fun, as is a walk around the 3rd Street Promenade. Venice Beach is just a bit further south.
Have you been to the Natural History Museum next door to the CA Science Center? That's a great museum, to.
You could go to a pro sports events - Ducks and Kings for hockey (Ducks are right there in Anaheim), or the Clippers or Lakers for basketball. You could also go to a UCLA game for a variety of sports (or USC or UCI, but I'm a Bruin so I'll say UCLA!). Walking around the campus at UCLA is really nice too. If you're in Westwood (by UCLA), there are great cookies for 35 cents at Diddy Riese and really good build your own pizzas at 800 degrees.
Knott's Berry Farm has tons of roller coasters and isn't too far from Disneyland.
Medieval Times is a great place for dinner with a live show. Choose the Buena Park location: http://medievaltimes.com/ It's just down the street from Knott's.
You could do the San Diego Zoo or Wild Animal Park. If you're here long enough, you could take a day trip to Catalina, though I'm not sure how much there is to do in the winter.
There is an Outdoor Education Center at Irvine Regional Park. I'm not totally sure if you can just go anytime, but it's worth looking into. They have archery, BB gun shooting, zip lining and more. Give them a call and see if you can make solo (non group) reservations. http://iroec.webs.com/
Tour the USS Midway aircraft carrier docked in San Diego
Drive out to Palm Springs and take the aerial tramway to the top. There might be snow up there. Stop to see the giant dinosaurs in Cabazon along the way. Big Bear and Idyllwild are also great for local snow trips.
Visit the Watts Towers