The Center on the Developing Child at Harvard University offers lots of great information online, including these short videos about early child development and the importance of two-way interactions:
From the very earliest weeks, you can "serve and return" with your baby: 5 Steps to Brain-Building Serve and Return
Play is healing and wonderful for all ages - and especially important for building healthy brains, starting in infancy. Here's a playful reminder: Building Babies' Brains Through Play
A 5-part video series (total about 30 minutes) about the importance of the early years in child development: Child Development Core Story
Helping parents notice the ways they're already interacting with their children, Dr. Philip A. Fisher encourages more of these brain-building interactions. Parenting for Brain Development
Also see Family and Home Network's article "Children's Needs".