We also have a lot of acreage, so when my boys came along we ended up putting a short picket fence, that's around 32" high, around a portion of the yard that's connected to the house. We made the fence ourselves, it took a while, but now I know that they won't run off. I still go outside with them and keep an eye on them but at least they can be on the deck with me. I wouldn't go with the invisible fence, my dad's neighbors have it and they have Jack Russels, and they do find ways to occasionally get out. Not safe enough for me.
...Theresa & The Boys 