Pro tip: Buy the Istanbul museum pass online – costs 85 TL, valid for 5 days. It saves time as you don’t have to stand in the queue to buy tickets, you can visit Hagia Sophia, Topkapi palace and 10 other museums on this pass. Even if you just visit these two, it is still worth it as the individual tickets are 40 TL each. Website: https://www.muze.gov.tr/tr/purchase?t=5
Basilica Cistern – The Basilica Cistern is the largest of several hundred ancient cisterns that lie beneath the city of Istanbul. The cistern provided a water filtration system via aqueducts for the Great Palace of Constantinople and Topkapi Palace. Fifty-two stone steps descend underground into the depths. Inside it is cool, dark, and you walk over bridges and walkways around water tanks. There are some big fish in the water! Look for the Medusa pillar, which featured in Dan Brown’s novel ‘Inferno’, and in the movie. Pro tip: It’s a good place to visit in the afternoon, as it provides some respite from the heat.