Hand out bottles of water to workers on a hot day; of course be careful not to hold up traffic.
Pay for someone's toll or hamburger, in line behind you at a drive through. (If you are worried about how much this will cost, just say, I'd like to pay $5.00 of the cost of the meal for the person behind me, or please give the person behind me a free ice cream cone, on me and tell them to have a great day!)
Get some travel size toiletries and donate them to a hospital. The hospital I volunteer for is a Children's Hospital and they accept all kind of supplies, new toys or stuffed animals, toiletries for parents who have to stay overnight such as shampoo, deodorant and tooth paste/brushes. Call your local hospital and ask what you can do.
Have a bake sale and donate the proceeds to your favorite charity.