Someone mentioned going abroad in the off season. A great opportunity is to go WWOOFing (WWOOF = "Willing workers on organic farms"). Many small non-profits and nature preserves are part of WWOOF and are looking for people to do trail maintenance, education, etc, and occasionally research or monitoring work. Its typically $40 to join WWOOF and the local chapter in the country you want to travel to, plus a plane ticket - the rest is usually between free and $5 if you're volunteering in the off-season at a farm. I've WWOOFed with my wife on farms Costa Rica, China and India and we've had great experiences at all of them.
Nathan
