A ton of Dr. Seuss based activity ideas, crafts, and lessons can be found at Hubbard's Cupboard.
Homegrown Hearts has great learning activities based on Horton Hears a Who.
What's your favorite Dr. Seuss book? Kids can take their own survey and record their findings here.
Don't let the lack of images fool you. There's a lot of great lessons on this site that is conveniently organized by book.
FiveJs had put together a great resource of teaching topics that work well with various Dr. Seuss books. It's organized by both topic and further down, by book title.
We have been having a great time with the Green Eggs and Ham Cookbook. We've made Goose Juice and Biggel balls so far, but there's so many more recipes we want to make!