Ein Gedi is the most famous area for tourists wanting to visit the Dead Sea and is super easy to get to by public bus from Jerusalem. It is a full day trip though so be prepared.
We got off the bus at the Ein Gedi spa rather than the public beach since it had a restaurant and proper services. I think the spa pass cost about $40 and was totally worth it. After floating in the sulfur spa a litte we headed out to cover ourselves with the mineral rich mud the area is known for. Then it was a simple matter of heading over to the little tram that takes you down to the Sea and back.
Its super fun floating around in the Dead Sea like a cork and the salt in the water gives the water a super slick feel. make sure you take some crappy rubber flipflops or crocs though! The ocean floor is covered in super sharp salt crystals not soft sand. It really sucked walking out into the water as soon as it was deep enough for me to float away I did so. So your going to want to avoid getting the water into your eyes and mouth. Really you have no idea just how salty the water is. If you get the water in your eyes its going to suck.
Masada is also on this bus route so if you are down there its a good idea to visit it. But be aware that Masada closes at 4:00PM. By the time we got there it was 3:00PM and we only had enough time to ride the cable car up, run around a little and ride the last car back down. So if you are planning on doing both either spend less time at the spa, or go to the spa after Masada.
