I was there a few days ago, and went to La Rambla at 8 am, while it was still cool enough to walk (it was very hot in the afternoon).
Deserted, and very few things were open. Not many places open for breakfast.
I thought I'd walk through the Gothic quarter, seeing the streets and squares while it was quiet. It was not nice; the shops were still closed and the shutters covered with grafiti. It would look better with the shops open.
But wat I didn't anticipate was the mess, the people sleeping in doorways (looked like drunk young people rather than homeless) and the general odour of urine in the streets. There were a lot of people sleeping rough in the streets near La Rambla.

The market was open, though, and not crowded, so I was glad I went that early.