Lindy really likes any kind of long strap or string type of thing (dog leash, jump rope, diaper bag strap, etc.) She can spend a long time manipulating it different ways, looking at it, and putting it in her mouth. At six months, maybe her favorite "toy" was the drawstring on a pair of my shorts. (Obviously, this kind of thing needs to be played with only under close supervision.)
Plastic grocery store bags are another big source of entertainment. Another thing that obviously requires close supervision. (Before I had a baby, I always thought it was kind of funny that bags had that "not a toy" warning - like someone might actually think it looked like a good toy. Little did I know . . .)
At about six months, Lindy also started to enjoy using my sitting or lying down body to try climbing and crawling on (with my breasts as her main goal.)