When my DS was younger and he would play army with his friend, they would take coke cans and hid them around the yard. They called these their bombs and missles. Maybe, since your in a warmer climate, you could do a scavenger hunt type thing. I know my DS and his friend would have thought that was kind of cool. Also, I just ordered from Oriental Trading some plastic army hats....they were pretty inexpensive but maybe you could find something along these lines so they kids can "dress the part".
If I had been courageous enough to have a home party, army would have been our theme. Have fun!