Lifehacker.com has a great list of "10 ways to entertain young children for $1 or less (without the TV)."
Our son and his around-the-corner friend communicate by walkie-talkie, so I'm not sure the tin can string and telephone is going to fly, but I'll give it a try.
As for the part about catching fireflies, does anyone know of a good place in Western New York to find them? I grew up in New Jersey, where fireflies seemed much more plentiful than here. Not that I'm complaining -- many kinds of bugs were more plentiful in my home state, also.