I know someone who's been experimenting with the same rough idea, but trying to be a bit proactive about things by putting together small kits of food/water/stuff (granola bars, raisin packets, water bottles, gift cards for chain restaurants, socks, toiletries) and keeping a few in the car to just hand out when he gets asked.
This is in LA, where you're usually in the car when you see unfamiliar people (down on their luck or not), so YMMV in places like SF where foot traffic is more the rule and you may not be habitually carrying lots of extra baggage.
Carrying a Subway gift card specifically to give away might work out, though. Maybe pair it with the card of addresses for local help institutions (shelter, food, addiction treatment, mental health if we still have any of these) for extra mileage.