I can only speak to what I did with both of my children - read to them, play with educational toys with them (math & alphabet based toys, etc.). Just enjoy this time with them while trying to teach them the basics, and more importantly how to learn.

That’s the best thing you can do to prepare them for learning at this stage of development, because more than anything you can teach them that learning and understanding new things can be both fun and rewarding.

If they reach school age only knowing that “learning” means an authority figure trying to drill things into them from a workbook, they’re going to be much more likely to not want to learn in the first place.