You could always use the Metromover and head into Mary Brickell Village too. Metromover is the lone useful form of public transportation we have here in Miami. I have heard great things about Oceanaire and another restaurant who's name escapes me but I know there is a "Lolita" involved haha.
The Gusman is a great venue. I saw Buddy Guy perfrom there and it was an incredibly good time.
Ocean Aire Seafood Room
900 South Miami Avenue, Miami, FL 33130
There are not many walking-distance dinner options downtown.
La Loggia is about 3 blocks west. They have decent Italian food.
Il Gabbiano and Prime Blue Grill are in a new building downtown at Biscayne & SE 3rd St. I've heard very good things about Il Gabbiano (Italian) and OK to good things about Prime Blue (steakhouse/seafood).
La Loggia Restaurant
68 W Flagler St, Miami, FL 33130
335 S Biscayne Blvd, Miami, FL 33131
Prime Blue Grille
315 S Biscayne Blvd, Miami, FL