I love pressed sandwiches! You can butter the outside, or you can also use a very very thin coat of mayonaisse to get a crisp crust also. Some of my favorites are:
fresh mozzarella, basil pesto, and sliced tomatoes
sliced turkey, carmelized onions (or sweet onions), bacon, and mustard (or mayo)
grilled chicken breast, cheddar cheese, arugula, and sun dried tomato spread
seared mushrooms and onions, grainy mustard, and gruyere cheese.