Since it’s already past midday, I’d start slowly at Grand Bazaar in Beyazıt rather than trying to rush straight through the monuments. Get there by tram to Beyazıt-Kapalıçarşı or Çemberlitaş—both are easy walks into the market lanes. Give yourself about 1.5 hours to wander; this place can feel like a maze, but that’s half the fun. Look for hand-painted ceramics, copperware, tea glasses, lanterns, and spice stalls tucked into side aisles. Prices are very negotiable here, so don’t be shy about bargaining, and keep an eye on the time because some sections wind down earlier than expected in the evening.
From the bazaar, it’s an easy walk down into Sultanahmet Square, which is the best first-day orientation point in the city. You’ll have the Hagia Sophia on one side, the Blue Mosque on the other, and plenty of room to just absorb the scale of the historic core without committing to a full museum stop yet. Spend about 45 minutes crossing the square, sitting for a tea if you want, and getting your bearings. Then head into the Blue Mosque—remember to dress modestly, and women should bring a scarf for covering if needed. Entry is free, but prayer times affect visiting hours, so expect a short wait if you arrive during worship; outside of those moments, the courtyard is one of the nicest places in the city to slow down and appreciate the symmetry and tilework.
For dinner, go up to Seven Hills Restaurant in Sultanahmet, which is one of the easiest “first-night” wins in Istanbul because the view does most of the work for you. It’s especially good around sunset and into blue hour, when the silhouettes of the Blue Mosque and Hagia Sophia make the whole skyline feel almost unreal. Plan on about 1.5 hours and roughly $25–40 per person, depending on whether you go for meze, grilled fish, or a full Turkish dinner with drinks. If you can, ask for a terrace table in advance—views fill fast.
If you still have energy after dinner, finish with a calm walk to Hagia Irene nearby in the Topkapı/Sultanahmet area. It’s usually much quieter than the major headline sights, which makes it a lovely way to end a first day without more crowds. Depending on the hour, it may be best to treat this as a short, reflective visit rather than a long stop—about 45 minutes is enough to take in the atmosphere and the layered history. The walk back through Sultanahmet after dark is one of the nicest in the city: the streets settle down, the tram lines feel simpler to navigate, and you’ll already have a sense of where everything sits for tomorrow.