I vary my meals seasonally (Spring/Summer/Fall/Winter), but overall I have about 5 themes that I vary: soups, pastas, casseroles (or what I call one-dish dinners), tacos/burritos and proteins (poultry, meats, fish, beans and the occasional tofu). During the Spring/Summer I have some entrée salads that I add to the rotation. An example of how I vary one of the menu themes is Fall/Winter soups: vegetable chili, black bean chili, white bean and turkey chili, beef stew, vegetable soup, chicken noodle soup, split pea soup, and broccoli cheddar.
I have about 10 meals that I often repeat throughout the month. Most of those are main dishes that feature a protein. These are dishes that I know my family usually enjoys. I start with those when I am writing my meal plan and then fill in with seasonally inspired dishes (lighter ones in late spring/summer, soups and stews in winter), vegetarian dishes, and those meals that I plan to purchase prepared or through take-out.