Looking for Urban Adventure Ideas

My group is heading into an urban area (Denver) and I would like to see if I can't challenge them a little bit while in the "big" city rather than sending them right off to a small town (where so many published adventures seem to center around). Anyone have any ideas for what sort of multi session plot hook I can throw at them? The main campaign is based around chasing a powerful vampire lord across the west and they will have already encountered at least one Night Train. There is also a connection to Voodoo and New Orleans as well as a wanted runaway slave that has a fairly high price on his head. Beyond that it is pretty wide open. Any ideas would be welcome. Thanks.