The two best materials I can think to use would be either colored sculpting clay or a styrofoam ball. The advantage of the sculpting clay is that you can use different colors of clay to highlight and differentiate different portions of the brain. For example, you can use four different colors to show the four major lobes of the cerebral cortex (frontal, parietal, occipital, and temporal). Different textures and features can be shaped into the soft clay.
The advantage of the styrofoam ball is that you can easily cut it in half to show an inner cross section of the brain, therefore showing and highlighting more areas of the brain like the thalamus and hypothalamus. You can carve these different features right into the styrofoam using a sharp blade like an exacto knife. You can also color the different sections of the brain using paint for added visual appeal.
Other materials you could use include paper mache, but this is rather messy and I'm not sure how creative it really is. Now if you really wanted to get creative and interesting, you could use some kind of cooked pasta for the outer surface of your model to get the right look and texture for the brain tissue. Perhaps some kind of spray glue or adhesive to keep the pasta shaped and in place. The obvious drawback here is that using food does bring up potential spoilage issues over time!
In my experience, making edible models always gets you brownie points! (Get it, brownies?) Anyway try making a jello brain with some twizzlers inside to represent nuerons. You can buy brain shaped molds at some party stores or online.
After you've got a spherical base, certain pastas (like macaroni) are the perfect shape and texture to create the various folds in the brain.
Or, if you're craftsy, you can knit or crochet or knit a brain. I've linked to two good patterns. The second one is even free online!
Good luck with your project!
When I have had to make spherical models before, I used things like basketballs and fruit (it goes bad, so you have to turn it in and trash it quickly). To get the texture of a brain, I would recommend using paper mache or clay. When I made a model of a cell, my group decided to make it all out of edible materials (hence using fruit as the base) and that turned out great. We used different kinds of candy, like Mike&Ikes and chocolate, to represent different things. Our teacher and classmates loved it because they got to eat it when we were done. Good luck!
In my health class we all had to first take different colored clays to represent the different lobes and other parts of the brain. Afterwards we also had to construct our own from home. Some people did baked goods, including me (I made a cake), but people were very creative. They used multiple materials such as pipe cleaners, beads, different candies, etc.
For a styrofoam model, a really easy way to do it is by using the image I included. By coloring the inside differently you will have a full model of the brain but also show the structures inside such as the hypothalamus and other parts.