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.