With beef I cut up onions (quarter them and keep them big and chunky) and place the beef on top. Sometimes I might put some canned tomatoes on the bottom (diced) but probably no more than 1/4 of a can.
Unless you are making something that should have a gravy (like a stew) you don’t want much in there and even with a stew most of the liquid is added much later, closer to the end of cooking. The crock pot really pulls the juices out of meat and veggies and the higher the water content the more liquid it will pull (and that doesn’t mean the meat will or should get dry – if it gets dry it was cooked too long). And of course, the cardinal rule is never, ever lift the lid! Unless the recipe calls for it or it’s near the end of cooking and you are adding say a can of corn or peas that just need to be heated through. Every time you life the lid you need to add another hour of cooking.
ETA: I’ve never cooked chicken breasts in the crock pot but are they boneless/skinless or have the skin on? If they have the skin, ball up some tin foil and put it in the bottom and then put the breast on top, that way the fat drips down and stays off the food. Got that tip on the Atkins site!