i used to really like marmite sandwiches- you could probably make them 'something' and marmite to make it a bit more filling though. ketchup sandwiches also went down well in our house, despite being pretty unhealthy, as did sliced boiled potatoes, pickled beetroot slices (make the bread go pink) and chip butties.
how about growing your own sprouts/cress (kids like that, and its not super hard) and adding it to sandwiches made with grated carrots, shredded raw brussels sprouts, and a bean spread or home made cashew cream cheese or peanut butter or something? there are loads of different nut butters around.
doing the sandwich itself differently might help with the battle. you could make pizza slice sandwiches too (like french bread pizza) or flatten a spice of bread, put a topping on it, and roll it into a swiss roll, and cut into spiral disks- make toast sandwiches into soldiers and use them for dipping, cut them out with cookie cutters, etc.
how about falafels in pitta bread? toasted fruit and jam sandwiches (careful, fruit gets really hot) are nice too. frozen, thawed, marinaded and pan fried tofu slices are brilliant in a sandwich or wrap too.