I did exactly the same trip from Delhi. Had a great journey on that train with nice folks who shared their fruits and laughs.
When I arrived in Agra I did the Taj Mahal tour alone took the usual photos then spent most of the day eating ice cream in hotels and roaming about.
The very day I went it was around 46C and far too hot to do much else. Plenty of ice cold Kingfisher beer before heading back to Delhi.
I visited Varanasi many times. Whenever I was in Lucknow for any length of time I'd always get to Varanasi for a couple of days. It was about a day's train ride.
Used to make a point to take photo's at Kedar Ghat.
A couple of times I walked for a good 2 hours in the summer heat but just couldn't find any end to all those Ghats.
I took some photo's at one place where there were a good half dozen fires burning bodies. Bit gruesome to be honest. Not something I wanted to experience seeing body parts fall out of the fire and being poked back in by a some ugly chap with a big pole.
Historic place. Bit of a S***h**e to be honest hehehe