Two separate parts to your question. How to cook the salmon and what to make the sandwich with (in addition to the salmon).
I suppose you can cook the salmon any way at all and then make a sandwich. Least favorite way for a sandwich would be poaching or en papillotte.
If you can't grill it, consider pan frying or broiling. You may marinate the fish in a favorite marinade then cook it. Pan frying or broiling will give you a nice browned crust for added flavor.
Choose the bread you want and dress up your sandwich. Consider scallions, roasted red peppers, tomato or whatever. Make a flavored spread from mayonnaise and different seasonings to spice up the deal.
Just use your imagination. Walk down the aisle in the market and take a good look at all they have to offer that could make an interesting sandwich topping.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan