Would they be interested in learning about those things from someone other than you? JL Collins’s stock series was enormously helpful for my understanding of investing when I started out. You could give them the book the “Simple Path to Wealth” (the edited/published version of the series) and it would be zero effort on your part. Bonus: he started writing it to teach his teenage daughter about investing, so the target audience would be perfect for your nieces (making some assumptions re: ages).