Mice should be fed a high roughage/grain diet, just think about what they would eat in the wild. Stuff like grain pellets, grass seeds, nuts, wholegrain biscuit etc should form the bulk of their diet. Fruit and vegetables should be given very sparingly as the ones with higher water content like cucumber or watermelon can cause diarrhoea. I used to give mine treats of leftover cooked wholegrain rice, peanuts, crust of high grain bread and stuff like that. Mice aren't like humans, they don't rely on fruit and veg for health. Very sparing amounts are great to mix up their diet and strictly as a treat, but they don't technically need them in big amounts. There is no '5 a day' for rodents. Hope this helps.