Build large casinos and gambling halls to bring in lots of tourists and increase revenue. These buildings can greatly generate traffic in your city, so make sure your mass transit is up to par. Gambling also leads to an increase in crime; ensure you have a police force large enough to combat it.
There is key information in the System Info of the Gambling SubMenu in the City Specialization menu. View the number of tourists visiting your city per day, and the amount of profit they generate for your city.
While on the Gambling SubMenu of the City Specialization menu, use the Data Map button to open the Tourism map and view where tourists are located and where they tend to gather.