This isn't a trick question. It might just happen, so you'll want to be prepared.
Do you remember any of your old hobbies? Have what you'll need handy -- a book, your knitting, the Wii -- so when some unexpected time becomes available, you won't waste it looking for hobby tools.
Another good way to spend some unexpected down time is with a little quick cleaning. This doesn't sound as much fun as the hobby, but if it reduces the time you have to dedicate to cleaning (how many hours do you spend cleaning over the weekend?), that can be a blessing in itself. The cleaning should just be a quick spruce, something that takes less than 15 minutes. Are there some dishes still in the sink? Can you give the dining room table a quick dust? Even just folding the sink towels or fluffing the couch pillows can improve the look and feel of your home. As with the hobbies, keep some cleaning tools right where they might be needed, so you can get to them quickly.
You might also want to think about getting in some quick exercise. The Centers for Disease Control (CDC) recommends Americans get at least 30 minutes a day of exercise, but you don't need to get all 30 minutes at once. Take a walk around the block or climb the steps to the second floor a few times.
If you find most of your down time comes while sitting in traffic, you can use this time well. Your CD player gives you a number of options: put in a book or start learning a foreign language. If your kids are in the car with you, play some classic car games: 20 questions, I-Spy or the license plate game.
Of course, not every minute needs to be filled. If you do find you have an extra 10-15 minutes with nothing to do, you can close your eyes and just relax.