I am fairly new to the sport, but spend a lot of time outdoors. Going along with what torry was saying about new locations, state land has a ton of good caching spots, though they are filled with hunters in the fall. They offer a wide variety of difficulty ranges and fairly large areas.
Also, it seems that you could keep a constant supply of fresh caches if you could have "re-cacheable caches" In which the the owner gives the cache a lifespan (a week, a month, etc.) and the first person to find the cache after the lifespan is up, gets to take it and hide it someother place in that area. But like I said I am fairly new to the sport, and don't know if this would be a popular idea.