I have LOTS of experience in this area after numerous hit-and-runs when I was parked outside.
First, you might want to take a moment to think just how hard you were hit. Remember that bumper cover, as the name says, covers up the stuff that might save your life in a serious crash later on. If you have any doubt, remove the cover and have the bumper inspected! If you're good at DIY body work, you can probably find a new bumper in a junkyard for a few bucks and a bit of your time. If not, you can graft some closed-cell foam shipping material onto the place where yours is crushed. (If there is metal damage, let a body shop handle the work. If you want to let granny off the hook, take the "at fault" hit on your own insurance.)
Back to the bumper cover. If the damage has gouges or other damage that significantly distorts the flat smooth surface, you'll need to either replace the whole bumper cover, or do some el-cheapo repair work. When I found a couple of gashes on my Mustang's bumper cover, I used a Dremel tool to grind down the raised portions, and filled in the valleys with thick, brush-on paint from a factory touch-up kit. I did it in layers, and used an old credit card to smooth out the excess. After the layers built up enough to form a slight lump, I sanded it down to flush, then used factory-color spray paint to get a solid color over the larger area. After that dried, I wet-sanded using crocus cloth, cleaned the whole area thoroughly and applied two layers of clear coat.
I can see the flaws easily, but most people who look at my car and don't know about the damage rarely notice.
