The Qala'at al-Bahrain Bahrain Fort is located off the northern shore and is a five to ten minute drive away from Manama city. It is restored and in good condition although it lacks furniture, signage, or exhibits. Admission is free.
Next door to the fort is a museum, completed in February 2008, which contains many artifacts ranging from the ancient Dilmun periods through the Islamic era, many of which were found at the fort and additional ruins next door. The museum is a large rectangular and white building with absolutely no signs to indicate that it is a museum. The hours are 8AM-2PM daily; admission is free.
Tree of Life. Although trees grow in Bahrain, this one is special because of its location in the middle of the desert amidst the oil wells and other infrastructure of the petroleum industry. You need a car to reach the tree, as it is far from the main roads and not on any public transportation route.
To reach the tree, take the Zallaq Highway heading east, which becomes the Al-Muaskar Highway. You will eventually see a sign for the Tree of Life indicating a right turn. Although the sign seems to point you to turn onto a dirt road which actually goes nowhere, do not do so, instead wait until the next intersection which is several metres ahead. There are no signs as you travel down this road, but pay attention to a scrap metal yard on your right. Before you reach a hill which warns you of a steep 10% incline, take a right. As you continue straight down this road including roundabouts, you will begin to see Tree of Life signs again. The signs will lead you down a road which will then be devoid of these signs, but you will eventually see the tree in the distance on the right it is large and wide, not to be mistaken for other smaller trees along the way. You turn onto a dirt path at Gas Well #371. You can drive up to just outside of the tree, but make sure you stay on the vehicle-worn path, as turning off of it is likely to get your car stuck in the softer sand.
Although it seems like a chore to reach, the Tree of Life is worth the visit for the oddity of it. The tree is covered in graffiti, although this is not visible until you get up close. Try to make your arrival near sunset for a picturesque view of the tree and the surrounding desert.
[Update 2011] There is also a new ? much higher quality road.
Bahrain also has a set of remarkable prehistoric burial grounds. These extensive sites, often densely covered by burial mounds, can be found at A'ali, Al Hajar, Buri, Hamad Town, Jannusan, Sa'ar, Shakhoora and Tylos.
There are a number of famous houses which can also be visited. Al Jasra House is located in Al Jasra village and was built by the late Amir, Shaikh Isa bin Salman Al Khalifa in 1933. It is an excellent example of Bahraini architecture. Bin Matar House is located on Muharraq island. It was constructed in 1905 by a successful pearl merchant, Salman bin Hussein Matar. It was subsequently used as a majlis.