Well, outside of using actual walkie talkies.+1 for Skype. Probably the easiest solution.
Did I miss something?
When did skype or even fring get walkie talkie features? I know they are VoIP phone apps but walkie talkies?!
"Walkie talkie" in the sense that you can use them to talk to each other, be it over wifi or 3g. Or, with actual walkie talkies, radio frequencies.
I think HeyTell would probably be your best bet.
I don't know of any local wifi apps, most of them have to go through the internet and back again.