Masjid Al Akbar Surabaya is one of Indonesia's largest mosques, a modern landmark with a broad blue-green dome and a minaret that rises above the southern side of the city. It is a very different religious sight from the older Ampel quarter, and that contrast is useful.
Visitors come for the scale, the patterned interior, and the tower view when access is available. Dress conservatively, avoid prayer times if you are only sightseeing, and remember that this is first of all a working mosque.