In my apparent quest to make every multiplayer board game a fun single-player experience, I came up with a solo mode for Carcassonne.
OK, it's not an original creation. I stole the basic idea from this BoardGameGeek post. I did make a couple of changes to make it play more smoothly.
If you've played Between Two Cities, the 7x7 grid works like the size limitations there. You can build outward in any way as long as you don't place a tile that would make your island 8 tiles long in any direction.
In normal Carcassonne, there is a rule that tells you to discard the tile you've drawn if you absolutely cannot place it on the board. In dozens of normal multiplayer games, I've only seen this happen once, and it was with the all-city tile. With the new size restrictions, though, you are much more likely to find pieces that can't be played.
End-game scoring works as normal. You cannot complete features that run into the edge of the island, but you still get partial credit for them at the end of the game.
Once you reach 7 tiles wide or 7 tiles long, it may help to place a different color of meeple on the outside of the edge where you can no longer build. I also use a third color of meeple as 50 points for every time my score wraps around the board.
This is a fun way to pass the time. Carcassonne already plays fast and this makes it even quicker. You can get away with things you couldn't otherwise, like stacking lots of knights in a single city or leaving features empty until the end of the game.
I got 130, 151, and 181 points as I started feeling more comfortable with the size restrictions and strategy. In my fourth game, I got 79, but I was one tile away from 250 points!
I have only played this with vanilla Carcassonne. I've only seen a couple of Carcassonne expansions in my life, so I can't say whether they would work well in this mode. You'd probably want a bigger island size to fit the expansion pieces in — a 7x7 grid gets you through maybe 3/4 of the base pieces.
The linked version requires you to keep the starting tile in the middle of the island, but allows you to discard a playable tile whenever you want. I think this isn't as fun and also harder to keep track of, since the starting tile does not look special from the front. Consider it a variant if you want to play that way.
I thought about adding in score penalties for missing tiles in the final island, or for unused meeples. In the end I decided against this. Technically, the only new rules in this mode are the 7x7 limitation and the 4-discard rule, so it's cleaner to leave penalties out. Plus, you will usually score higher if you fill the island and use all your meeples by the end of the game, so it felt wrong to double-penalize you for missing.