Good for you for thinking ahead. You can look in the Food section below for lots of recipes, or use the search function, also to search ina particular section like Low Carb for "casserole" etc. if you are following a specific plan.
If you haven't got anything prepped, don't forget meals whose main component involves opening a can -- if I haven't got something I always head for a can of black beans or refried beans (check for the vegetarian/low fat kind). Shred some lettuce, open the salsa jar, pop the beans into the microwave, some lowfat cheese shredded on top, sliced avocado if you have it. Greek yogurt is a good sour cream substitute. I usually have some tortillas in the freezer but usually I don't need them.
In most of the crockpot recipes you can leave out a lot of the added fat. The important thing is if there is any meat in the recipe to brown it before you put it in the crockpot, it makes it tastier. I have found some great crockpot ideas on this website: A Year of Slow Cooking
. In the contents there is a "Light and Healthy" section but you can modify a lot of her regular stuff. If you don't have a lot of practice with a crockpot, make sure to read her FAQs like "help my slow cooker cooks too fast" in the Popular Posts on the lower right.
In the end though because I have a GIANT crockpot (didn't have many choices when buying) and live alone, I usually only pull it out for guest dinners, and instead on the weekend make casseroles and portion them out for the freezer. This weekend I made this: Cabbage Beef casserole
to use up some stuff that was lying around in the fridge and cabinets. I used wild rice instead of regular rice (make sure to precook the rice if you do this) for more fiber, added a lot more herbs, and because I didn't have any beef broth, used a tip I think from kaplods and put in a can of tomato juice.
If you see a casserole recipe that you kind of like but is pasta-based, think about substituting grilled thinly sliced vegetables like zucchini or summer squash for the noodles to reduce the calories. Other substitutions: yogurt or evaporated milk instead of cream or canned cream of etc. soup, or in other recipes chicken or beef broth or even a bit of beer or wine.