I like to go down to the river when it's running high and shoot at whatever it brings me, which can be anything from aluminum cans or plastic bottles to basketballs and cans of spray paint. When the river's up, it's moving along at probably a 4-5 mph pace, so you don't have all the time in the world to aim and have to lead things a bit.
Orange spray paint makes a nice orange ball in the river.
Orange spray paint makes a nice orange ball in the river.