I work in a small bookstore in a small town, so lots of regular customers that I've gotten to know over the past couple years. The other day, one of these regulars came in- really sweet lady, a retired teacher, super chatty, but she always has interesting things to say, so you don't really mind. Anyway, she was browsing and we were chatting and exchanging notes on books, when she picked up a copy of Bird by Bird by Ann Lamott and asked if I'd ever read it. I said no, I hadn't, and she said "Oh, you must! It's the most wonderful book, everyone should read it!" I said I would definitely add it to my list. Anyway, she chooses another couple of books, comes up to pay, I ring her up and start to put everything in a bag, but when I get to Bird by Bird, she stops me and says "No, that's for you! You keep it and just read it and tell me what you think next time I'm in here." It was so sweet of her and just put me in a good mood for the rest of the day, especially because I hadn't been having the best week and I never buy myself books because I'm too broke. (And yes, I did start reading it, and yes, it is very good!)
TL;DR: Sweet retired teacher performs best random act of kindness ever and buys me a book at the store where I work.