I like to make it a fun timeline study while we watch them incubate. There really isn't anything to see so by making a timeline it shows us what is going on in the eggs. I found this idea a few years ago while researching how to incubate so I can't give credit to whomever had this idea as I have no idea where I found it.
(here is our timeline and incubator)
We take 21 plastic Easter eggs and number them 1-21. I look for embryo pictures online and cut them out and put one picture in each corresponding egg. We make a 21 day timeline and hang it on the wall behind the incubator. Every day we open the egg that corresponds to that day and hang the picture on the timeline. No matter how many times we've done this, the kids still love it! Here they are filling the eggs with the embryo pictures.
Here are some really good sites for learning about incubation and for the embryo photos:
Mississippi State's 4-H Club - great site for black and white photos of embryo and lots of great information
Univ. of Illinois - Incubation and Embryology - cute site for lesson plan ideas (we're trying the egg in the bottle trick)
Univ. of Nebraska, Lincoln - 4-H Embryology page - some good information but I don't like their embryo pictures
Enchanted Learning - lesson plans, worksheets, embryo pictures and more