The Federation is shown to be able to fully control the weather of planets; I expect that Earth would be one of those planets. I could see United Earth making the Colorado River more regular.
We know the Southwest US is more populated than now, with cities like Mohave that turned the desert to more lush parkland; this is where Christopher Pike is born. However, even if you consider the region to remain water stressed, most of Las Vegas currently is water efficient; I would expect the local government to continue with efforts to treat sewage in order to make it potable water again.
As for urban planning, I expect The Strip would remain the focal point of Las Vegas, flanked by high rise buildings down a wide avenue. Likely, the sprawl outside of The Strip gets replaced by higher density apartment buildings.
Given that poker is still routinely played in the 24th century, there are likely several other games that survived as well. As you said, you probably have people gambling over bragging rights, so I expect more table games and less slot machines.
