If you are in Manama, you have to visit the Grand Mosque (also known as Fateh, Al Fateh or just the mosque). It's great and it's enormous. It's about 100 meters wide and 80 meters long - it can easily accommodate about 7000 people, you can probably do 10000 or more, easily.
Also, it's fairly modern being built in 1988 it's one of newer mosques but it's also beautify. The main hall prayer hall is also super big, it's airy and it's pristine.
Al Fateh Mosque is a must visit location for any tourist that visits Manama and Bahrain - just be respectful to all people and it really does not matter if you are a Muslim or not - just come and visit Al Fateh Mosque.