Oh noes! A player can’t make it for some reason! You don’t want to continue the main story and game and leave them out, so what can you do? What’s a good substitute for a regular game session?
1: Something Else set in the same World
Some games are properties with many games and interpretations. Some have boardgames, card games, computer games that you can play together and more. Doing this you can keep the players in a similar mindset and inhabiting the game world until the next session, with a bit less of a gap and disconnect.
2. A One-Shot
If the game is simple enough, you could run off a one-shot, simple adventure and some new characters, just to have something else in your specific iteration of the game world. You can even import whatever happens there, or even the characters you have created, into your main game.
3. Do some world and character building
Go over the character backgrounds, their place in the world, figure out more about them and how they interface with the game world. This can give character specific background, side events and knowledge for when you do get back to the main game.
4. Any other game!
Play something else. It’ll scratch that itch and bide you over until you can play again.
5. Take the L, and Use the Time
Cancel the session, apologise to everytong, spend the extra time to make what you have planned for the next session even better, tighter and well planned.