1. Students can make their own hats.
2. Play a game of Fish, Fish, Cat (Duck, Duck, Goose)
3. Discuss rainy day activities. Ask students to draw/paint what they enjoy doing on a rainy day.
4. Make fish kites
5. Make a paper plate cat.
6. Make these gorgeous Cat in the Hat posters.
7. Write a summary/book review like the ones found on Art Smarts 4 Kids.
8. Try these free colouring pages from Early Moments
9. Have a magnetic fishing game already made. You'll need cardboard (possibly laminated) with paper clips or safety pins attached. You'll need some 'fishing rods' with magnets. You may want to cut out a fish bowl to place the fish on.
10. Make a kite like the one that Thing 1 and Thing 2 have in the story or get students to design their own. Go outside and fly them before hometime.
11. Ask students to draw themselves on a ball balancing several things at once.
12. Have some things for students to try balancing e.g. balls, small bean bags, a plastic cup etc. Allow them to hold things and let them try placing these things on their head or feet.
There are some more great ideas at http://lilteacher.com/seussville.html. Scroll down to "The Cat in the Hat'
Have fun with it.