A must-see, the Dead Sea is beautiful and fun, but be careful when swimming. At 30% salinity, nearly 10 times that of the normal ocean, the Dead Sea is the saltiest place on Earth. The water level is also dropping a meter every year-- meaning the water will only get saltier.
The best way to get to the Dead Sea is with a tour, but there are also bus lines that go directly there. Check Egged for busses, and you can book tours from Abraham hostels, Tourist Israel, Fun Time, or other companies. Keep in mind there won't be many busses; the area isn't bustling.
Swimming in the Dead Sea
Before you go:
Try to avoid shaving for at least three days before entering the Dead Sea, and try to avoid having any open wounds (blisters, cuts, etc.) Make sure to take plenty of water with you. If you have water shoes, you may want them-- the salt on the bottom of the lake can be sharp. Flip flops are not recommended, as they will get stuck in the mud.
Entering the Dead Sea:
Walk in slowly, careful not to splash. When you get to about waist deep, simply sit back, and you will easily be able to float on top of the water. Be extremely careful not to get the water in your eyes, ears, nose, or mouth. If you do, have a friend lead you to the fresh water showers commonplace at public beaches. Do not put your head underwater; floating on your back is preferable in order to avoid this. Do not stay in the water for more than thirty minutes, and afterward, wash all the salt off in freshwater showers or your skin will be affected. Be sure to hydrate and flush the salt from your mouth.
You can take a mud bath in the world-famous Dead Sea mud, which contains four times as many minerals as normal ocean sand. You can feel the mud on the sea floor. The extremely slimy stuff is the best. It's safe to smear it all over your body, and don't forget to snap a pic! Be warned that the mud may stain light colored swimsuits, and if you are fair skinned, it may tint your skin orange.
Masada is an ancient Judean fortress as well as a UNESCO World Heritage Site. You may read more about the history here.
You can ascend the mountian via the cable cars or via the Snake Trail. If you are physically able, it is highly recommended to climb the Snake Trail. Get there at 5:00 (or earlier) to see the sunrise. Climbing any time after 8:00 will be absolutely miserable because the area is hot and the trail is not shaded. The hike is not easy and will take about forty minutes. The trail quite literally snakes up the mountain, and involves many steps. There is an entrance fee, but you can get a discount with your MIT student ID. Make sure to carry water with you, at least a liter. There is a place to fill up at the top.
Masada may be accessed via a tour or via Egged busses.