This is a quick and fun build- I've had the idea to set up a zip line ever since we had fun on a zip line at a picnic last summer. Zip line trolleys, the part that skims along the cable, retail from "stupid" to "insane" so I picked up some pulleys at McMaster (thanks again, Sportbike), some construction ties and hardware at the Home Deep, and started assembling.
I attached the two ties to a chunk of 2x4 before drilling so they'd result in a mirror image, and line up nicely. The pulley axle holes are 3/8" so I bought hardware to match. Then a quick monkey swing: traced the bottom of a bucket onto some leftover plywood, jigsawed and sanded, then a hole drilled for the rope. I had 50' of 1/4" cable so I attached a thimble and clamps to have loops in the ends; I roped one end of the cable to a tree and used a cable puller as a tensioner at the other end.
All in, about $20 for the cable, maybe $10 for hardware, and $13 for pulleys (I forget what shipping was from McMaster, but reasonable). Smile on 6 y.o., priceless. Next step is to get a 100' or better cable, because 50' is just too short!