I really enjoyed Warsaw. My hotel was next to the Academy of Arts and Sciences. There is a nice subway system that goes through the main parts of the city. Old Town Warsaw is also very beautiful, make sure you visit the Royal Castle
Prague/Praha is by far the most beautiful city I have ever been to. Make sure to see everything including Prague Castle and the Charles Bridge. A must visit on any trip to Czech Republic.
If you are going to visit Riverside, make sure you visit Mission Inn or the Riverside Art Museum. Downtown is really the only place worth visiting.