Muscat

Al Jalali Fort and Al Mirani fort
They are located in Qasr Al Alam Street. The forts were built as prisons in the rocky mountains in 1580 during the Portugese occupation, and have now been converted into museums. Update as of August 2012, the forts are now closed to the public. However, outside photography is allowed.
Qasr Al Alam Royal Palace
Guarded by the twin forts of Jalali and Mirani, this is the office of Sultan Qaboos, the ruler of Oman. This beautiful palace stands on the head of a natural deep water harbour. Visitors are not allowed to visit the palace, but they are allowed to take photographs at the entrance of the palace.
Corniche Area
The recently renovated Corniche area is a popular place for a walk and also for its many eating places. Climb to the base of Mutrah Fort at the east end of the Corniche walk for a spectacular view of the city.
Sultan Qaboos Grand Mosque
This is the third largest mosque in the world and mostly the entire complex is open to non-Muslim visitors. Non-Muslims may visit from 8AM to 11AM every day except Thursday and Friday. Ladies are however expected to keep their heads, ankles and wrists covered while visiting the mosque. Must sees in the mosque include the Swarovski crystal chandelier, the second largest hand made persian carpet in the world and the marble panelling.
Bait az-Zubair Museum
The museum itself features displays on Omani social history, while tours are frequently run of the neighbouring reconstructed townhouse

Muscat also has a large number of small and large parks, the largest being the Qurum National Park or rose gardens which include a large manmade waterfall, a lake and an amusement park which is a must to visit during the Muscat festival. Other popular parks include:

Riyam park
also with rides and an Arabian watch tower. Visited in The Amazing Race 9

A number of factories are open for visitors such as the Omani halwah factory and the Amouage perfume factory. Amouage perfume is the most valuable perfume in the world.

Nakhal Fort
This splendid fort is about 40 minutes drive east of the city, located at the base of the Jebel Akhdar section of the Hajar Mountains.
Wadi Shab
This is perhaps the most spectacular of all the easily accessible wadis in Oman. It is located about 100 km southeast of Muscat, accessed from the main coastal road to Sur at the village of Tiwi. The pools here are a vivid Emerald Green colour, and the caves and sheer sides of the wadi really dramatic.
museums
Bait Al Zubair

Bait Muzna Gallery

Children's Museum
Shatti al qurum
Marine Science & Fisheries Centre
between the Al Bustan Palace Hotel and the Capital Yacht Club.
National Museum
near Abdulridha Mosque-Ruwi.
Natural History Museum
in the Minstry of National Heritage and Culture. Al Khuwair
Omani French Museum
Near muscat police stn, Located at old Muscat in Bait Fransa.
Omani Museum
Medinat Al Alam Information City near Ministry of Information.
The Sultan's Armed Forces Museum
Bait al falaj.