There are two series of books that I'm thinking of for today's post. First, I guess, isn't really a series, but an author - Phoebe Gillman. (She is one of my favourite children's authors/illustrators.) Secondly, the Little Critter Books by Mercer Mayer.
These books have one thing in common - secrets hidden in the pictures. If you look closely enough, and you know what to look for, there is more than what one initially sees.
Gillman skillfully hides characters and objects from her other books, in her illustrations (excepting The Balloon Tree, because that was her first book). Finding these pictures is satisfying by tricky, as the searcher needs to know her other works, well enough to recognize a familiar character or object. The ability to find something for which you don't know you are looking, requires some skill and higher level thinking, so this would be a better activity for older children. Or with help from an adult, they may memorize where the pictures are and then find them on their own.
Mayer's books have a cricket and spider hidden on each page (mostly). They are fairly easy to spot, so I would recommend this for younger children. Even by the age of two, once your child knows what they are looking for, they should be able to spot them on many of the pages.
It's just a fun addition to reading books and it's something your child can do alone if they are not yet able to read themselves. Happy hunting.