There's a lot of video of him and he seems cheerful enough. However, something happens to Riley on video we can't forget, he hit his head. Obviously, not enough to cause injury that would show in his autopsy, but that coupled with things he ingested could have really impaired his thinking process. I mean, i don't know that, but i just think it's worth considering.
I can't find a rational explanation for going to the river, i really can't. Seeing it is usually people's clue that they have gone too far (especially on foot). So, people usually turn around and go back the way they came. There's not like there's some fun little dock to explore or anything like that.
I just keep coming back to maybe he was trying to go somewhere (not his hotel). It either had to be someplace he heard about and wasn't sure where it was (which it might not even be there anymore, maps are not always up to date). Or he was trying to meet a person that he had never communicated with via phone/text. That sounds super unlikely these days, so idk about that.
I don't think we are ever going to find a logical reason for this unless the police release a bunch of new clues. (ETA: i don't really expect that to happen, but i am just saying it would be more information.)
ETA 2: i was just thinking about Riverfront Park. Okay, it's really just this nice little area where you can look at the river and kind of hang out. But, if there's not an event going on there (he passes it in the video, there's nothing going on there), it might not look like much to someone from out of town at night. So, maybe he was looking for it? And didn't realize he found it? It's perfectly safe from a drowning standpoint: you have a nice view of the river, but you're really not that close to it.