If you like tile placing mechanic, Carcassonne is a God among the games. And if you are also a solo player like me, you must have tried all solitaire variants available on the net, and must have your favorite one.
Well, for me it is Famine in Carcassonne by p4warrior. Rules can be found on BGG here or as Google document here.
This variant is based on upkeep mechanics which is great since it is more challenging and surprisingly more similar to multiplayer mechanic then other proposed solo variants. Another good thing is that it needs almost no bookkeeping.
The only problem I have with this variant is that it supposes that the game ends with the win at the moment you put down the last tile. So, the more tiles you have the game gets more and more difficult. For example I play with all light gray tiles I have, and that is a lot of tiles :)
So, to solve this issue, I decide to house rule this variant a bit.
My suggestion is that for a win you have to harvest following:
3 knights from the city at least 3 tiles big
3 thieves from the road at least 4 tiles long.
This way a player gets the clear goal to strive to, and according to whom it can guide the tactics during the play.
To keep track of harvested maples, you can use scoring track and big maples from Ins and Cathedrals expansion, red for monks, green for peasants, yellow for thieves and blue for knights.
If you try this variant with my end game rules, let me know what you think.