You can plan your trip this way
Day 1: Go on a boat ride, you'll get to know about all the ghats of varanasi.
Day 2: Visit Kashi Vishwanath Temple, Annapurna Temple and Kaal Bhairav Temple. Time To visit this place depends on the crowd.
Day 3: Visit New Kashi Vishwanath Temple, Tulsi Manas Temple, Sankat Mochan Temple, Durga Temple and Tridev Temple. This would take 2-3 hours. If you have time you can head towards sarnath where you can visit Sarnath Temple, Tibetan Temple, Chinese Temple, Japanese Temple and Sarangnath Temple