Kelloggs has a promotion on many of its cereal packets at the moment offering a free ticket to zoos and aquariums (aquaria?) when accompanied by a full paying adult.

Worth noting too that not all zoos and aquariums are signed up for the offer, but there's a pretty good selction, with around 40 or so on the list, including London Zoo, The Living Rainforest, Bristol Zoo, Paignton Zoo, Amazon World, Colchester Zoo, Drayton Manor, Dudley Zoological Gardens, Twycross Zoo, Woburn Safari Park, Whipsnade Zoo, West Midlands Safari Park, Blue Planet Aquarium, Chester Zoo, Curraghs Wildlife Park, Deep Sea World, Edinburgh Zoo, Belfast Zoological Gardens and Dublin Zoo .

The offer on the pack seems to be valid 'till 15th Jan 2009.

Here's the link to the Kelloggs website 

For more offers, discounts and where to find free tickets to attractions and venues, check the IdeasForTheKids Discounts to attractions forum.