Its pretty much a create your own sandwich machine. For breads, try challah, sourdough, ciabetta, even baguette. Cheeses that melt well are most cheddars except for the really aged ones (like over 3 years, they can melt but not usually to the consistancy people like), havarti, younger gouda, most italian cheeses that are similar to cheddar in hardness or softer like asiago, provolone, fontina, etc, swisses that are in that same vein (not too hard). You can also use crumbled blue cheese, feta, or goat cheese. Lots of deli meats work, but you can also use left overs like rotisserie/baked/grilled chicken or turkey, you could even slice up left over steak from a restaurant. For other ingredients, if your grocery store has a salad bar or mediterranian/olive bar, you can find lots of stuff on there for paninis. Roasted or sundried tomatoes, roasted peppers, chopped olives, marinated artichokes.
So for instance thinking of all the stuff at my grocery store...
Ciabatta bread, then mix some of the oil from roasted or sun dried tomatoes into a little bit of butter and spread it very thin on the outside (like for a grilled cheese) then on the inside some fresh baby spinach leaves, fresh tomato slices (or some of the roasted ones) mozzerella and provolone, and some sliced cooked chicken.
Sourdough bread, swiss cheese (something like Jarlsberg or Emmental), sliced smoked ham, and fresh tomato.
A play on a philly cheese steak, french bread, swiss cheese, some caramelized onions, and sliced left over steak.
For something sweet, cinnamon raisin bread, with some brie spread inside with sliced apples or pears inside, then drizzle very sparingly with honey.
Bread, roasted peppers, spinach, some chopped up olives and crumbled goat cheese.
I mean the possibilities are really only limited to what you like to pair together.
