The short answer to this question is EVERYTHING!
Your child should be reading everything he/she can read. This includes reading cereal boxes at the breakfast table, reading toy descriptions in catalogs or on websites, reading news articles of interest in the paper or online, reading and rereading favorite picture books, reading street signs, reading grocery lists, reading food labels, and so much more.
If your child gets used to reading everything he/she can at a young age, he/she will become a reader who wants to learn and know more about all kinds of things.
Reading should not just be a homework assignment where a timer is set and reading must be done. Reading should be ingrained at an early age as a fun activity and something that helps you learn more about the world around you.
How do you do this? Start reading to and with your child as soon as he/she is born. Read board books and let your child turn the pages. Read books with lots of pictures and ask your child questions about what he/she sees. Model reading for your child. Your child should see you reading for work or for pleasure many times a week or even many times a day. Have a family reading night where all electronics are off and everyone can enjoy a good book on their own or as a family. Read chapter books to your kids. Keep the suspense going by stopping in the middle of a chapter and picking it back up the next evening. Read to your child each night before bed.
Again, what should your child be reading? EVERYTHING!