I think it's nice when another driver gives you that little 'thank you wave' when they pass, even though they had right of way anyway (say if the parked cars are on your side so you're waiting behind them)
Or when another mother pushing a pram will give you a little smile when you're walking along pushing your pram.
Or when someone gives you their parking ticket because they're leaving and there's still some time left on it.
Or when the delivery man sees your cat and says he's lovely.
What other nice things do you like that strangers do?