You don't need any documents to get into Juarez but you do need to pay an immigration fee of less than a dollar. One DOES need some proof of US citizenship to get back in, although most of the times I've come back in I haven't been asked to show anything.

The easiest way to get to Juarez is to park near the bridge in downtown El Paso and walk across, to where there are buses and taxis to be had. There is also a trolley car on wheels that leaves from the El Paso convention center and makes a loop through Juarez, stopping at all of the natural tourist sites. People who pay to ride the trolley can just hop on and hop off.