On the back of the Squares bars there is a voucher for free entry (with a separate full paying adult) to Alton Towers, Thorpe Park, Legoland and more so essentially half the ticket price.
These are harder to find in the summer so best to get them now. Valid until 30th June 2017
More info at http://www.kelloggs.co.uk/promotions
Unsure if they are on other Kellogg's products