My husband and I took this cruise about 15 years ago, though we've traveled to Puerta Vallarta several times since. It's our favorite town! The cruise line should give you a list of on-shore tour packages OR there are usually locals at the dock that offer great possibilities.
In Mazatlan we took a tour into the "countryside" for a tour of a furniture factory (very rustic), followed by lunch at a Cantina and free time in a small town housing many artist's shops. Mazatlan is also known for Fire Opals and the shopping is fabulous if you're in the market for a piece of jewelry.
Puerta Vallarta is a wonderful town and home to "Gringo Gulch" (where most Americans lived many years ago) and Mismaloya Beach (Night of the Iguana filmed there). I understand there is a place to go zip-lining just outside of town (it wasn't there last time we visited). Shopping is great, just go a block or two off the main drag, less touristy and more reasonable prices. Don't pay the 1st price quoted, they love to dicker. You'll find just about anything you could be interested in PV as far as shopping. Prices on .925 silver are generally really good. There's a "below street level" open market style shopping area near the south end of town that's quite fun and some really nice shops a little closer to "Gringo Gulch". If you love horseback riding, that's available as well. We rode out into the countryside to the local river (and drinking water source) and quickly saw for ourselves why NOT to drink the water. Our ride ended at a ranch just out of town where we enjoyed a wonderful BBQ dinner. Stay away from citrus or anything veggie that needs to be washed while on shore, unless the establishment can confirm they use bottled or purified water.
WE spent half a day in Cabo and the ONLY activites (it was a Sunday) were a tour on a glass bottomed boat or sitting at a bar on the beach. We chose the former and it was quite fun. I understand there's more to do there now but not sure what.
As to dressing up...by all means do. It's great fun to dress for dinner and really a shame that some consider regular beach attire appropriate for a formal dinner. I have acquired several evening gowns for such evenings and it's always fun. Macy's had quite a collection on display recently and usually their prices are reasonable.
For more ideas, consider getting a travel book covering the Mexican Riveria.
You'll have a great time!