This is a very common question, and I've used lots of different ideas before. I've even shared some of them here.
But if you would like to have some academic fun with the little ones, there are tons of free printables on the web. My friend Tiffany turned me on to Homeschool Creations (which I'm sure 90% of you already knew about). There is a wealth of free printable resources for toddlers and preschoolers, and even for your younger elementary kids.
Since I have a 3yo and a 5yo, I typically school these two together with Five in a Row. However, I also like to use book or unit studies to fill in between times. And because these two are boys, it's easy to do topics that appeals to them both!
Next week, we'll be doing some fun activities from Cars, thanks to the fun Preschool and Tot Packs at Homeschool Creations. There is also a Toddler Pack called Monster Trucks, and who could leave out the Fireman, Astronaut, and Policeman packs?
If you need some fun stuff to add to your preschool and you need it to be free, head on over and see all the good stuff Jolanthe has created!