My eldest son, now finishing 10th grade, took calculus at RSM this year and the AP Calculus BC exam at school. He likes programming and intends to major in computer science. What have your children done or considered doing in math after calculus, before college? Are there online courses you recommend? Here are a few that look interesting:

Harvard EdX Calculus Applied!: Apply tools of single-variable calculus to create and analyze mathematical models used by real practitioners in social, life, and physical sciences.
(The class applies the concepts of single-variable calculus but does not reteach them. This could give him ideas for a computational science fair project.)

Imperial College London Coursera Mathematics for Machine Learning Specialization consisting of
  • Mathematics for Machine Learning: Linear Algebra Mathematics for Machine Learning: Linear Algebra
  • Mathematics for Machine Learning: Multivariate Calculus
  • Mathematics for Machine Learning: PCA
(He is interested in machine learning.)

AOPS Calculus (Has anyone taken this after a standard calculus course in school? I assume it would cover things in more depth.)

Stanford Pre-Collegiate University-Level Online Math and Physics has the following math courses:
  • Multivariable Differential Calculus
  • Multivariable Integral Calculus
  • Linear Algebra
  • Modern Algebra
  • Real Analysis
  • Differential Equations
  • Complex Analysis
  • Partial Differential Equations
  • Elementary Theory of Numbers

Coursera Introduction to Discrete Mathematics for Computer Science Specialization has the following courses:
  • What is a Proof?
  • Combinatorics and Probability
  • Introduction to Graph Theory
  • Number Theory and Cryptography
  • Delivery Problem

Wesleyan Coursera Introduction to Complex Analysis

KAIST Coursera Introduction to Ordinary Differential Equations