What are the must-see sights in Istanbul and how much should I budget for sightseeing?

In the Karakoy/Beyoglu area, check out Orthodox churches: Holy Trinity Church (Aya Triada) is the largest one nearby, with several other churches in the same district — easy to find on maps.

Trip.com works well as a booking aggregator for tickets. Note: the Basilica Cistern is NOT included in the Istanbul Museum Pass, and they haven't accepted cash for over six months — card payment only.

Extended Istanbul route: Dolmabahce Palace, Camlica Mosque, Blue Mosque, Rustem Pasha and Suleymaniye mosques, Buyukada island, Holy Trinity Church, St. Anthony of Padua Church, and Ihlamur, Yildiz and Kucuksu palaces. For food — budget-friendly Balkan Lokantasi, mid-range Hurrem Sultan, and try breakfast at least once on the Seven Hills terrace for the panoramic view. Budget $650 for comfort, though $500 is enough.

Core sightseeing list for a multi-day visit: Hagia Sophia, the Archaeological Museum, the Dolmabahce/Yildiz palace complex, the Basilica Cistern, and the art museum next to Dolmabahce.

Beyond the famous sights, it's worth stopping at churches along the way: Holy Trinity Church (Aya Triada Rum Ortodoks Kilisesi) and Santa Maria Draperis Church — both are conveniently located along the typical tourist route.

The Basilica Cistern is open for visits, confirmed in person — payment is card-only.