We have a picket fence like the one in Brittany's photo above. If you use a picket fence, make sure your baby cannot fit through it. My 9 pound boys can fit through ours. My dad didn't believe me until he saw Teddy put his shoulders through while I had him on leash.
Teddy could have slipped through even easier when he was younger and narrower.
You could add wire fencing from the garden center -- just make sure it doesn't have any sharp edges.