Especially for the ones for whom yarn bombing takes too much time (by the sizes needed), Mail Art is the way to go!
You can knit and send lighthouses, snowmen, envelopes, or simply knit.
You can weave or even crochet mail art cards (thank you again, Thom!).
Some months ago I rediscovered Spool Knitting (also known as Punniken, Tricotin, Strickliesel, リリヤン, Caterinetta, Bobentrikilo - anyone who knows the word in other languages?).
While creating, suddenly this Snail Mail Snail shown above appeared.
Below there was a new snail-to-be born (temporary put on a public transport chair, here) but suddenly I decided to give the result to a little girl who was watching very interested and who might be our mail art colleague within 15 years or so :-)