The important districts in Uttar Pradesh's Braj Circuit are Mathura, Vrindavan, and Agra. Let's explore each of these districts in detail:

1. Mathura:
Mathura is a significant district in Uttar Pradesh and is considered one of the most sacred places in Hinduism. It is believed to be the birthplace of Lord Krishna and is therefore highly revered by devotees. Mathura is known for its numerous ancient temples and religious sites. Some of the prominent attractions in Mathura include:

- Krishna Janmabhoomi: This is the site where Lord Krishna is believed to have been born. The temple complex houses the prison cell where he was born and various other shrines dedicated to him and his consort Radha.

- Dwarkadhish Temple: Dedicated to Lord Krishna, this temple is a major pilgrimage site for devotees. It showcases intricate architectural designs and hosts vibrant festivals throughout the year.

2. Vrindavan:
Vrindavan is another important district in the Braj Circuit and is closely associated with Lord Krishna's childhood and youth. It is believed to be the place where he spent most of his time playing and performing divine activities. Vrindavan is famous for its numerous temples, each dedicated to a different aspect of Lord Krishna's life. Some of the notable attractions in Vrindavan include:

- Banke Bihari Temple: This temple is dedicated to Lord Krishna and is one of the most visited temples in Vrindavan. The deity of Lord Krishna here is known for his playful and mischievous expressions.

- Prem Mandir: A recently constructed temple, Prem Mandir is known for its grand architecture and intricate carvings. It depicts various episodes from Lord Krishna's life and is illuminated beautifully at night.

3. Agra:
Though not directly a part of the Braj Circuit, Agra is in close proximity and is often included in the tourist itinerary due to its world-famous monument, the Taj Mahal. Agra is a historically rich city that was once the capital of the Mughal Empire. Some of the key attractions in Agra are:

- Taj Mahal: This iconic white marble mausoleum is a UNESCO World Heritage Site and is considered one of the Seven Wonders of the World. It was built by Emperor Shah Jahan in memory of his beloved wife, Mumtaz Mahal.

- Agra Fort: Another UNESCO World Heritage Site, the Agra Fort is a massive fort complex that served as the residence of the Mughal emperors. It showcases a blend of Islamic and Hindu architectural styles.

In conclusion, the important districts in Uttar Pradesh's Braj Circuit are Mathura, Vrindavan, and Agra. These districts are famous for their rich religious and historical significance, attracting millions of tourists and devotees every year.