Milan is a beautiful city with a lot to offer, even if you only have one day to explore. Here are some suggestions from various travel blogs and websites:

• Piazza del Duomo & Duomo Terraces: The Duomo is Milans most famous landmark and a must-see attraction. You can climb to the top of the Duomo for a stunning view of the city.

• Galleria Vittorio Emanuele II: This is a beautiful shopping arcade with high-end stores and restaurants. Its located next to the Duomo and is a great place to take a break and people-watch.

• Castello Sforzesco: This is a castle that houses several museums and art collections. Its a great place to learn about Milans history and culture.

• Parco Sempione: This is the largest park in Milan and a lovely spot to relax and enjoy nature. Its located next to the Castello Sforzesco.

• The Last Supper: This famous mural painting by Leonardo da Vinci is located in the refectory of the Convent of Santa Maria delle Grazie. Its recommended to book tickets in advance as they sell out quickly.

• Brera District: This is a trendy neighborhood with narrow streets, boutiques, and art galleries. Its a great place to explore and get a feel for Milans local culture.

• Arco della Pace: This is a triumphal arch located at the entrance of Parco Sempione. Its a great spot for photos and a nice place to relax.

• Navigli District: This is a neighborhood with canals, restaurants, and bars. Its a great place to visit in the evening for a drink or dinner.

• San Bernardino alle Ossa: This is a church with a unique ossuary decorated with human bones. Its a bit macabre but also fascinating.

These are just a few suggestions for what to see in Milan in one day. Its important to note that Milan has a lot to offer and its impossible to see everything in one day. Its recommended to prioritize what you want to see and plan accordingly. Additionally, its a good idea to book tickets in advance for popular attractions to avoid long lines and save time.