For example, infants are most interested in movement and sound. So, shaking a brightly colored rattle or a key ring will stimulate them. Bright colors, shapes, animals' picture, would be also much attracted to their attention.
When they grow a little order, textural toys they can touch or squish in their hands, such as stuffed animals, would be recommended.
During the age of to months, we can help our children to create "real life" activities into play, such as drive car, have a tea party, etc.
The most important thing we should bear in mind is that while playing, our children are happy or not. So, paying attention what interests to our children would be much helpful.