Philae Temple is dedicated to the goddess Isis and dates back to around 690 BC, though the entire structure was dismantled and moved to its current island location after the Aswan High Dam threatened to flood it.
It's only reachable by a short water taxi ride, which adds a bit of ceremony to the visit that Luxor's more accessible temples don't have. Aswan's other Nubian sites are nearby, so Philae works well as part of a half-day loop rather than a standalone stop.