A less structured preschool might help. I don't know how a "formal" preschool works, my kids have both gone through play-based daycare centers. At these centers there are many picture books (these are much more difficult than the beginning reader books). Kids can read any book they want, they can read on their own, or to a teacher, or ask a teacher to read to them. Same with other activities: they can choose math toys, puzzles, art projects, pretend play, building blocks, board games, or anything that is available. Teachers simply guide them on whatever they choose. The lack of structure might make some wonder what exactly the kids learned, but our experience is that it actually fostered intellectual growth really well.
