San Juan is a fast game with a small foot print. Perfect for solo play, but there is no official variant. Here is a solo variant where the goal is to beat your own high score.
1. Each round you choose one role. You may only perform the associated action for the role and do not get the special privilege. The Councillor role is removed and the Prospector role can be chosen, but there is no action associated with it.
2. At the end of each round, draw four cards. Keep one and discard the others.
3. Remove the Library cards from the game.
4. There is no restriction on the number of buildings that can be built.
5. When the deck runs out, you do not reshuffle the discard deck and continue. The game ends when you draw a card but there are no cards left to draw.
The game plays on the timer of the card draw. The strategy revolves around retaining as many drawn cards as you can, to use for trading and building, before the deck runs out.
The variant can be made easier or harder by changing the number of cards you draw each round. This alters the value of buildings that let you draw or keep extra cards as well as giving you more or less time to build.
The prospector action exists so that the Gold Mine may be used. The Gold Mine could also be removed from the game to avoid this technicality.