Khakis and a nice shirt should be fine. That's what I wore to the casual nights on my cruise. I don't own a formal suit, but for the formal nights, I wore dress slacks, shirt and tie, and sport coat. I'm one that likes to get dressed up (by my standards) on formal nights. By dressed up I mean what I wore to the formal night dinner.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)