Disneyland Resort: Weekdays in Jaunuary, May, September, and early December (excluding holidays)Knott's Berry Farm: Any weekday between Labor Day and Memorial Day, except during holiday periodsLegoland California: Any weekday when school is in session (check operating calendar...not open all weekdays in off season)Universal Studios Hollywood: Any day between Labor Day and Memorial Day, except Saturdays and holidaysSeaWorld San Diego: Any day between Labor Day and Memorial DaySix Flags Magic Mountain: Any Sunday during weekend-only operation (except holidays)
Yes, attractions will be closed, but unless you've never been to the park or you're going for one specific attraction, the low crowds make up for missing one or two rides (I'd delay a visit if three or more major attractions will be closed). However, at the seasonal parks or parks I've never visited before, I'd much rather visit in the middle of the season during daily operation and hope for low crowds. If it's crowded, I'll just buy the skip the line pass offered at that particular park (almost all major parks now offer some form of this).
Oh, and yeah rides go down for refurbishments. Just want to gloat about the current Indy closure. Please read the comments on this post from two years ago.