Most casseroles freeze easily enough. Pasta bakes with meat and spaghetti sauce is easy enough. So are enchiladas. I love chicken ones with green chile sauce and before I was due, I made sure to make and freeze several pans of them.
Meatloaves are easy, as well. They don't even need to be cooked before frozen, just smoosh the meat mixture into loaf pans and cover.
For the slow cooker, you can use gallon freezer bags and fill them with all the ingredients for a soup. Beef stew, chilli or chunky veggie are easiest, though I don't recommend adding potatoes - they go black if frozen long enough.
I love the idea of freezer meals and have tried it but my memory doesn't mesh well with the concept. I either forget what I have in the freezer and make something fresh, or I forget to pull something out early enough to defrost so I have to rustle up something fresh anyway.