Put your friend & family's addresses in something small and easy to carry with you (mobile phone, little teeny address book, PDA, whatever). Make especially sure you have the full addresses of any who are either under the age of 20 or over the age of 60.
If you've got old blank postcards around, grab 'em and a couple pens you like to write with. Buy more postcards if you need to. Or find amusing ones among those free advertising kind. Or just get a bunch of postcard size pieces of blank paper and draw doodles on them instead of the usual pictures.
Walk down to the post office and buy a bunch of postcard stamps.
Find a nice coffee or tea house and sit writing quick little notes to everyone, just saying hi.
Mail them and make sure you still have more postcard stamps to carry with you.
Next time you see a nice postcard – especially one of those free ones – grab it and take a couple minutes to brighten someone's day.
(Advanced tip for Hipster PDA users: make your cards postcard size so that you always have the ingredients for an impromptu hello)