In the near term, probably expand into Cuba and the Caribbean possessions the US took from Spain. In the long term, probably expand through Mexico and Central America all the way to South America that they had their eyes on.
William Walker managed to invade Baja California, MX around 1853 with 150 mercenaries to create The Republic of Lower California and Sonora. He also invaded Nicaragua in 1855 and seized control by 1856 and made himself President of Nicaragua. He was eventually ousted the following year by the U.S. Navy. There's a movie about him called "Walker" based on his exploits.
