Lately, I've been making fried egg sandwiches. They're awesome. So first, I put mayonnaise on a couple slices of bread. Heat your skillet to about 175°C or 350°F then cut the heat to a medium-low. Fry your egg however you want. I prefer over hard for a fried egg sandwich. I love over easy, but runny yolks and sandwiches don't mix, imho. Once the egg is finished cooking, take it out of the skillet, put the bread in the skillet mayo side down, put a slice of cheese (I use American cheese product because let's be real, it ain't real cheese, but hot damn does it melt good) on one of the slices and the egg on top of the cheese. This traps the heat and makes the cheese super melty. Use the spatula to check the toastiness of your bread. You want it to be golden brown. It takes probably 2 minutes to get there. If you want to go the extra mile, add a sausage patty on top of the egg.
I know mayo is a weird choice to some people. If you feel weird using mayo, use butter instead. But mayo will add a whole new dimension of flavor to the toasty side of your bread. You're missing out if you haven't tried it.
My go-to lunchtime sandwich would be a good old turkey and Swiss sammy on potato bread with mayo, black pepper, lettuce, and pickles. Pepperidge Farms makes this potato bread that is just *chefs kiss* AGH! C'est magnifique. A little expensive as bread goes but so worth it.
Havarti is some boss level cheese, hfs. A+ choice there.