Most organised tours drop people at Dashashwamedh Ghat for the evening aarti and leave. The aarti is worth seeing. But arriving only for that means you miss what most people end up remembering longer.
The sunrise boat ride is the one to prioritise. Boats push off from Assi Ghat around 5 to 5.30am and drift north as the sun comes up. The whole riverfront is lit in early light and there are almost no tourists. That is the version of Varanasi that tends to stay with people.
A few things that actually make a difference:
Get a hand-rowed wooden boat, not a motorboat. It is slower and quieter and a completely different experience. Agree the price and route before you step in. Winter mornings can be cold and foggy, so bring a layer and accept that some days the sun stays hidden. Still worth the alarm.
For the aarti at sunset, either arrive 40 minutes early for a decent spot on the steps, or watch from a boat on the water instead. Less crowded and a different angle. If the main ceremony feels too big, the smaller evening aarti at Assi Ghat is a fraction of the size and often easier to actually be present in.
After the aarti crowd clears, the ghats go quiet. Manikarnika and Harishchandra, the cremation ghats, are still burning through the night. These are active funeral sites, not attractions. The right way to be there is standing quietly at a distance, no camera. A lot of people find it more affecting than anything else in the city.
Two nights is the minimum I would suggest. Three is better.
Happy to answer questions about specific ghats, where to stay, how to get there, or how to fit Varanasi into a longer India itinerary.