In Varanasi, a few must do's:
1)A boat ride in Ganges in the morning as well as the evening
2)A cup of tea sitting on the Ghats
3)Kachodi-Jalebi in the morning. I never found it as great in taste as its made out to be, but still worth a try.Lavang Latta and Hot Milk can also be tried.
4)A ride on a cycle rickshaw across the BHU campus. Its really beautiful!
5)A visit to temples like Vishwanath Temple, Manas Mandir and SankatMochan Mandir.
6)If you are fortunate, you might get a chance to attend a live concert of some of the best known names in Indian Classical Music and Dance for free at SankatMochan Mandir or Dhashwamedh Ghat.I got the chance to see Bhajan Sopori,Late Kelucharan Mohapatra,Sitara Devi,Birju Maharaj,Bismillah Khan,Pt Rajan Sajan Mishra when I was in Varanasi.