In restaurants, train or plane, in a waiting room or at home, colouring is an activity that develops fine motor skills and creativity in a peaceful manner.  

Children can choose their favorite theme and print all kinds of beautiful pictures to colour in just a few clicks:  superheroes, fairies, Barbies, goblins, latest movies, knights and princesses, trucks,... Some pictures are basic, specific for toddlers,  some mandalas are very sophisticated and would suit teenagers.

Of course there are other colouring sites, but I find this one comprehensive and easy to use.