When I'm playing by myself, I like to play against the ghost. If I have paper and pencil handy, I'll play bowliards.
Playing the ghost in 9 ball, you break, take ball in hand and try to run out. If you fail to run out, you lose. If you run out, mark up a game for yourself. I play races to 7 or 9 against the ghost. Playing ghost is aka playing God, because if you miss, you lose.
Bowliards is a good way to track your progress. Rack a normal 10 ball rack (looks like an 8 ball rack without the last row, 1 in front, 10 in the middle). This game is scored like bowling. Break, take ball in hand, try to run out. If you run out, you get a strike. If you fail to run out, the number of balls you made equates to the number of pins dropped with a first ball in bowling. Take ball in hand again, and continue to try to run out. If you run out, you get a spare. If you don't, you get an open frame, equal to the number of balls you made in your attempts. Play 10 frames, just like bowling. Keep track of your scores (I have a notebook full of scores). This is a good way to see how you're progressing, especially if you keep scores for years.