Signatures tend to change the most, especially if they specialize in seafood: what is fresh and can be shipped in quick and cheap changes by the day.
Next come the vegetables: almost every restaurant will change their veggies and salads by what's in season (fruits, too). Almost ever place on property will have a steak; what is served with the steak will depend on what's in season.
Then, seasonal foods in general change. Squash isn't too hard to get all year, but you'll be much more likely to see a squash soup on Fall and Winter menus then on the Spring and Summer (likewise, cold soups generally disappear during the Fall and Winter). Pumpkin tends to be fall and fall alone, as so many people associate it with Thanksgiving and would get confused seeing it in say, July. Corn, on the other hand, doesn't usually appear in February.
Finally, chef's do like to play around, test their wings and play with what they have. If an item is particularly trendy, it'll probably pop up: kale was the 'it' vegetable for a while and showed up everywhere, before now just occasionally sitting around. If a certain crop does really well, it'll be focused on no matter what: I went to Disney one year when Pears were doing awesome, and it seemed like every place had a pear dessert. And, of course, no chef wants a restaurant that stagnates and never changed: new things will be added as things they don't think do as well, serve their purposes as well, or represent their restaurants as well quietly fade away.
So, basically: the main part will usually be the same, though it may come from different places, and seafood in particular is more likely to change. Sides, pasta dishes, and vegetarian dishes will change entirely. Entrees are more likely to stick around the appetizers and desserts, save for items that have become 'signature' to that place, and even they may be altered in small ways.