Green stuff will provide your mouse with vitamins. You can give your mouse different vegetables and fruit - salad, cucumber, peas, apple, pear... In order to avoid diarrhea, start with very small portions.
For a source of animal protein, dog kibble (dry dog food) a couple of times a week will do. Low fat non spicy and unsalted, cooked meat or chicken or dried fish are also popular. Giving too much animal protein may cause skin problems (rash), so don't overdo it.
Nice change in a mouse's diet can be provided by giving porridges, boiled pasta and rice or mashed potato. Natural yogurt mixed in will make a very tasty meal and give your mice calcium. I haven't seen any mice turn up its nose in front of baby food either.
For a source of calcium you can give your mouse a calcium stone or mix powdered calcium in soft food. Egg shells contain plenty of calcium. You can make a fun dish for your mice by filling up half an egg shell with porridge, mashed potato or some other similar food!
Gnawing is natural for a rodent and hard but tasty food is good for mice. You can dry bread for your mice. Cut the bread up in small pieces and let them dry. Make sure the bits do not get moldy! Dog biscuits are suitable as well.
You can use your imagination with mouse feeding. Just remember that it should get a diet which is suitable for mice and that the mouse has to get everything it needs from the diet. Do not give: salty, sugary, spicy or fried food. Sweets are not a part of a mouse diet, neither are potato chips and similar snacks. As a treat you can give small bits of yogurt drops. There are also all kinds of honey bells and seed bars available and you can attach them around the cage so that your mice will have to climb around in order to get to the treats.