I don't know about you, but I get tired of eating the same things day in and day out. I imagine my dogs do too so I try to put a little variety into their daily meals. I never get away from concentrate because it has all the balanced nutrition they need, but I spike their food a little and they love it. Here are some of the things I do:
fresh carrots: shred carrots into their food or dice them into little cubes. It can also be a snack between meals.
hard boiled egg: mash a 1/4 hard boiled egg with a fork and mix it into their food. Not every day.
rice and vegetables: make rice like you normally do (without salt) and add carrots, garlic pieces, green peas, green beans. Mix about 2 Tbs into their concentrate.
chicken liver pate: Take chicken livers and put them in a skillet with a little water. Let them boil for 20 minutes or until they are cooked. Mash them with a fork and put it in a jar in the refrigerator. Put 1 tsp of that mixture to spike their dry food and they will lick their bowls. You must use it up fast because it doesn't keep.
tuna fish: Instead of throwing away the olive oil or water out of your tuna fish cans, put it in a small jar and use to spike up your pet's dry food.
saltine crackers: You can give your dog a snack usings saltless saltine crackers. They love them.
Yogurt: give yourd dogs a little yogurt now and then.
chicken soup: Use the bones that are left over from deboning chicken and make your dogs some chicken broth to add to their dry food.