Highway: not a concrete object, but I take it as a symbol nonetheless. Highways are made out of dead material, there is no room for living things (such as grass), and they serve the purpose to go from point A to point B as straight as possible. In a way, Gunny Highway is or has become like that too. He's made of stone, nothing moves him, not even death of his own soldiers. There is no room for living things such as emotions, otherwise he wouldn't survive as a marine in war. The only thing he knows how to do is get his job done, there's no room for private life, compassion, such things (would be the detours of the highway, in my understanding)
Aggie's dresses: everytime Aggie is letting Highway close, she wears white clothes. When she picks him up after Grenada, her dress strongly reminds me of a wedding dress. So white may stand for wedding, love and such things, but white is also the color of the page where nothing is yet written, the color of a new start.
cigars: the cigar, by itself, symbolizes power, wealth, success, and smoking a cigar is supposed to show relaxation as well (as a non-smoker, I can't really understand this

). In the movie the cigar is also introduced with the words:
suck on one of these, smooth as a prom queen's thighs, only not quite so risky
which is a very very bold sexual statement.
First, in the scene mentioned above, the cigar is meant to bribe Gunny. He refuses.
Later, Stitch Jones smokes a cigar as a sign of power, a sign of being cool, master of the situation, when the platoon introduces the Swede to Gunny. The cigar is not to be seen when they all rush outdoors

Only interest Gunny shows in dead ennemy is the search for the cigar. When he takes it, it's like saying: "see, I am the stronger, I won over you, so the cigar is mine now". And, when he finally smokes it, on the top of the hill, there couldn't be any image more macho than that. The sexual phallic symbol is allied with the one of power.
(BTW, doesn't the canon over which they secure their rope to climb up the wall resemble a lying, thus useless and already defeated cigar?

)