Plum District is offering a deal today to the Aquarium of the Bay at Pier 39. Get an adult and child ticket, for $13 - normally $25. I've written about the Aquarium of the Bay several times (search the Aquarium tag below on the right, or check out this Oyster Locals post). I really like this place.
You can buy four vouchers per person, and you have to use it by 1/30/11. The kids need to be ages 3-11 to qualify as the "child" in this ticket. Get the deal here - today only.