Since Istanbul is a city famous for its historical and cultural riches, it can be difficult to visit all the important places in one day. However, for a short day trip to Istanbul, here is a suggestion of some of the popular and important places:

Morning:

Sultanahmet Square: You can start your day in Sultanahmet Square. This historical square contains important structures such as Hagia Sophia, Topkapi Palace and the Blue Mosque.

Hagia Sophia: You can visit Hagia Sophia, which is considered one of the largest and most important churches in world history.

Blue Mosque (Blue Mosque): Located right next to Hagia Sophia, this beautiful mosque is famous for its interior decorated with blue ceramics.

Lunch: 4. Grand Bazaar: You can go to the Grand Bazaar after lunch. This historic market is filled with hundreds of shops and stalls, where you can find handicrafts, jewelry, carpets, clothing and more.

Afternoon: 5. Topkapi Palace: You can visit Topkapi Palace, located near the Grand Bazaar. You can see the life in the former capital of the Ottoman Empire and the riches of the palace.

Gülhane Park: Located right next to Topkapı Palace, this park is an ideal place to relax and take a peaceful walk.
Evening: 7. Bosphorus Tour: You can take a boat tour on the Bosphorus in the evening or in the evening. This gives you the opportunity to see the city from the sea and allows you to enjoy beautiful views.

Taksim Square: You can go to Taksim Square for the evening or dinner. You can find different restaurants, cafes and entertainment venues here.
Notes:

Istanbul's traffic can be heavy, so you should schedule carefully.
The places listed above are only suggestions and it may not be possible to visit them all in one day. Remember to plan according to your needs and interests.
Be sure to check factors such as entrance fees, opening hours, and COVID-19 restrictions in advance.
It can be difficult to explore all the beauties of Istanbul in one day, but the above suggestions can offer you the opportunity to visit the most important places in a short time.