Water Wow, Invisible Ink type booklets are always a hit. A bit pricey, but spans a large age range, portable, aren't the typical plastic junk, and lots of themes/characters for any kid.

Pair with candy (gasp!!) like a ring pop, mini bag of oreos, etc.

And you're golden.

What age?

Are we talking 2.5yr olds or 5 yrs olds that just missed the Kinderg. cutoff?