There are some very good guided tours in Mazatlan; ones that visit colonial villages in the Sierra Madere can be very interesting if you want to get into Mexican culture. We did a walking tour of Old Mazatlan on our last trip that was very enjoyable. There are some that I think are real rip-offs, such as tequila factories, sandal and tile makers, glass blowers, breweries; while mildly interesting, they are little more than selling opportunities.
Two of the best things to do in Mazatlan are to take an open-air taxi to the Golden Zone for shopping, beaches and good food. The deluxe resorts like El Cid will let you use their facilities right on the beach for no charge. Or take a water taxi to Stone Island for great beaches and very reasonable shrimp feasts and beer.