Family photo album or single family photos. You could even use your smart phone gallery
One of Theo's favorite things is family so what better way to get those cognitive wheels spinning other than pulling out some recent (and older) family photos?
Recognition and being able to tell the difference between objects is a wonderful beginning to the active cognitive learning journey.
Later comes counting, patterning and more advanced sorting (shape, color, size, etc). If recognition of familiar people or things is too easy you can introduce color/object sorting of simple attributes.
You could also introduce rote counting (counting verbally straight through "1,2,3,4,5". If that is too easy you can introduce counting with one to one correspondence: pointing to each object as you count it.
Still too easy? Throw in that "ABABAB" patterning (ex: red, blue, red, blue). Since Theo is only 1.5 it was age appropriate that I stayed at family recognition both new and older photos.
It's not so easy for a 1 year old mind to recall most people, places and things!