This might sound really gross to some people but I got it out of a kids' cookbook when I was a kid and I really liked it. It's one of my kids favorites now.
2 slices of bread
2 slices of American chesse
2 slices of tomatos
Toast the bread on the light setting. Spread one side of each piece of bread with mayonnaise. Put a tomatoe in the middle of the bread on top of the mayo. Place cheese over top. Spread a thin layer of peanut butter over the top of the cheese. Put in oven broiler or toaster oven until the cheese has melted and peanut butter is bubbly. Serve while hot.
This really is good. If the peanut butter is too weird for you, just make the sandwich without it. The mayo and tomato make the grilled cheese yummy.