Central Rio has certainly changed a lot in the last few hundred years - I mean, it was a swamp back then. Now look at it. Bustling metropolis. It's definitely one of those places where it is interesting to look at even fairly recent (20-30 years) photos of it and see just how it has become so much denser and urbanised. In the centre itself there is only one park - the rest is hard and grey.
Rio's centre is a mix of fairly interesting colonial era architecture and soulless new stuff, with a few pieces of inspired modern Brazilian architecture thrown in. Possibly the most inspiring of these is the metropolitan cathedral, which is quite a sight to behold. Looking at it without knowing what it is, some people might assume it is some sort of massive industrial venting facility. It's only when you get right up close that you see in fact that it's a church, with seating for thousands, and with four stained glass windows that rise up the full 75m height of the building. Quite a sight. Modern and brutalistic and quite unique. Even the bell tower is external: the odd little structure by its side, with a bell on each level.
And a lot of central Rio is pretty run down. Even derelict in some spots. And homeless people. Lots of them, many of them with a wide eyed, animalistic, opioid derived stare. They are everywhere. Little rows of improvised cardboard tents alongside buildings with rough wool rugs on the ground. One man I saw had a duffel bag on his head. Perhaps to give a little bit of dark and quiet (headspace?) so he could sleep during the day.
Still. I didn't feel unsafe in the centre. I probably wouldn't have tried starting a conversation with the wide eyed addicts, but neither did I feel the need to hide anything and everything valuable as everybody recommended, and I felt perfectly safe wandering around after dark.
Also. Parking. One of the reasons I decided to use a scooter to get around is because I thought, like most cities, parking a scooter would be easy pretty much anywhere. I was wrong: there seem to be only a handful of dedicated bike parking bays in the city centre and if you don't know where they are, good luck finding them, and just randomly parking it does not seem to be encouraged. So, each time I visited the centre I parked up round the back of the main military headquarters a little way out of the centre - my scooter had an armed guard. How comforting.
One thing nobody can fail to notice in Rio is the graffiti: a lot of it is really very impressive. I'm not talking about the Olympic wall mural (which I neglected to go and see), instead I mean the everyday artwork which seems to cover every available flat surface - even inside many of the road tunnels. Lots of the graffiti is the same old artless tagging found everywhere, but much of it is artistic and interesting. I could have happily done a specific tour of Rio based around the graffiti, and anybody with an interest in street art has to visit.