It's true that there is no how-to book on parenting and many parents second-guess what they are doing, worried about whether or not their child is on target with average development. While it's important to keep in mind that every child develops at their own rate, there are developmental charts online that will help guide you towards activities that are age-appropriate and that will encourage your child to grow, learn and try new things.
Scholastic's Early Childhood Today site offers Developmental charts that are organized into seven areas - play, creative development, physical development, cognitive development, math/science development, social/emotional development and language/literacy development.
The charts can be downloaded for free, as PDFs. They break down each category into ages, what type of abilities will emerge and what activities you can do to encourage further development.
For example, cooperative play for children of 4-5 years of age becomes imaginative play, which includes plot and character exploration - changing their voices, playing dress-up and taking on different roles. Parents can encourage this kind of exploration by teaching them games like follow the leader, or helping them set up a restaurant or post office, where they can try different roles/dialogue/problem solving.