The variant played very smooth and easy. However I won both times I have played without even subtracting points from the AI for missed parks. Have you had similar results? I'll be thinking about how to make the AI more challenging.
Its been a year since I touched the game. From my game play notes, it appears that the AI was winning most of my solo games. To be honest, I only played a handful (under 10). To try to make it more difficult on the Human player, do not eliminate the 1/2 card value the AI did not win; or perhaps subtract from the AI score 1 point (or a set value) per card not won. That should increase the difficulty on the Human player.
Again, it is a very simplistic family rainy day fun game, IMO. I wanted to keep it this way, and change the mechanics as little as possible out of respect to the designers. So its a solo variant for a game that had none. You are welcomed to tinker with my variant or develop something completely different.
Best of luck!