While reading 100 great works of literature is great advice I used a simpler method with my children. (2 girls, now in their early 30’s and both successful)
While they were in elementary and high school:
1) My wife and I both read to them as did their baby sitter
2) Encouraged them to read the works of Shakespeare
3) Regularly took them to museums here in NY and around the country when we travelled (They actually developed a love of museums)
4) Worked with them to do 5 extra math problems every school evening so that they would develop real skills in mathematics
None of these took money but did require my wife and me to be active parents
Even understanding the Cliffs Notes version of most of these would be an improvement over knowing nothing.