Ah yes! Senseless acts of kindness and random crimes of beauty! A long time since I've heard of that. I sometimes put my parking ticket if it has time left on it on someone's car if their's has run out, let the kid with the candybar go ahead of me in line, pick up the money for the old lady who drops her change in line so she doesn't have to bend over and do it...another is to go ahead and pay for the poor person who doesn't have enough to cover the bus fare.
I tend to tell people that not to worry about paying back, just help the next person who needs it and consider the debt paid! That tends to go oer well with the immigrant folks here for whom saving face is important, but for whom money is a problem and who don't know me from Adam.
Another hint, if you do little things for strangers, you usually don't get stuck in the expectation trap, but you definitely can make two people's day; your's and theirs. Good luck and HUGGGGSSSS!