There is a very active cultural scene in Mazatlán. Check out the events here.
Miles of Great long sandy beaches
Great surfing spot
Deer Island Tours
Deep water fishing
Fresh water fishing
All kinds of water toys and sports
The Mazatlán Festival of the Arts in October
The Sinaloa Festival of the Arts in November
The Sinaloa Orchestra (here for Babblefish English Translation)
The Great Carnaval of Mazatlán -- third largest parade in the world.
Many dance and music events at Teatro Angela Peralta
Professional AAA baseball October to February one of the best leagues in the world
Bohemian-style night life in old Mazatlán every day!
One day out-of-town tours
A wide variety of night life
You can look at 360 degree views and see restaurants and other points of interests by taking an interactive virtual tour of Mazatlan here.
Check out a local paper called the Pacific Pearl. It covers some of the happenings around town. Also, you can visit Mazatlán Interactivo for more information on what is going on. Of course, Lonely Planet also has some great background information on traveling around Mazatlan and the nearby cities.
For a general calendar of events, Maz Travel has a pretty good calendar of events. Also, you can take a virtual tour of Mazatlán, its restaurants, bars, hotels and tourist sites here.
Mexico's Tourism Board also has a nice website in several different languages.