The Shephelah is characterized by many fertile hills, checkered with many ruins from ancient times. There are also a lot of small natural springs, which are almost always being diverted into ancient stone pools. A popular and famous hobby of Israeli teenagers is to repair and clean these pools or even build completely new ones. Most israelis from the central regions go to these pools to relax and cool off during the summer, and during the weekends its rare to be alone in those springs for a long amount of time. Its also common to see "chill out" zones and fruit trees (especually figs and raspberries) near those springs.
The Shephelah was a main and important region during biblical times. It is the gateway to Jerusalem and other big cities in the mountains, such as Hebron and Bethlehem, and there were many important cities in the Shephelah itself, such as Beit Shemesh and Gezer.
It was also the main battleground between the Israelites, who sat in the mountains, and their arch enemies the Philistines, a greek nation that invaded from the sea and conquered the coastal plains of Israel. One of the most famous battles in the world, the battle between David and Goliath, took place in Valley of Elah, one of two main valleys in the Shephelah (along with Ayalon vally).
During the israeli war of independence, the Shephelah was the main battleground between Israeli forces, who tried to break the road to Jerusalem, and the armies of Jordan and Iraq, who tried to reach the coastal plains and conquer the temporary capital of Israel, Tel Aviv. In the end, The line of truce between Israel and Jordan passed mostly through the Shephelah. During the six days war in '67, all of the western bank of the Jordan river was conquered by Israel, including the Jordanian Shephelah and the mountains of Judea and Samaria.
The Shephelah has beautiful views of the coastal plains from one side and the mountains of Judea and Samaria on the other side. Probably everywhere you will go you will have beautiful views.
As a central place in Israel since the days of the bible, the Shephelah has many marked trail routes that crosses the hills and valleys, while passing by springs and ruins. There isn't any organized reserve to pay and walk in a specific route. To travel here, you need to get a map of the area or to know exactly where your route starts, get there, and just start walking.
Like in the rest of Israel, each official trail is marked with 3 different colors, and you simply need to follow those 3 colors as they appear on rocks and trees throughout the route and tell you in which direction to go.
The Shephelah is very safe to travel. However, Though not as common as in the Negev desert, car theft near arab villages is not uncommon. If you travel by car to a place surrounded by arab villages, park in a jewish town (the Shephelah is populated mostly by jews, so chances are there is a jewish town near where you want to go).
If you go to a place that crosses the green line (the line between the west bank and the rest of Israel), It is highly recommended to coordinate your trip with the Israeli army, hire an armed medic, and keep away from large Palestinian cities. If you are going on a guided tour with another group, they will probably hire the medic and coordinate the trip for you.