One day off a cruise ship on Puerto Vallarta. What's the best to do?

Either hang out in the Old Town, eating, drinking, shopping, strolling the Malecon. Or go with Los Veranos for a zip-line trip. Very fun!!

Agreeing with the above... are you looking for a formal activity, some kind of tour? Or just wanting to see a bit of the city?

Personally I'd spend the day at Los Muertos (local public beach) relaxing, eating & drinking at a beach bar there. A bit of shopping on the south-side nearby ("Zona Romantica"). A walk around El Centro (downtown area) and visit the famous church (with the crown on the top). Have a nice lunch and nice dinner somewhere with a view. Definitely stroll The Malecon.

But you could do the exact opposite, heading north on the coast to some of the less populated beaches and smaller towns on the bus.

Or you could do any of many organized tours. Check out www.allvallarta.com or www.vallartainfo.com for ideas.

If you want a resort with a day pass, I found this one (suggested by the good folks at Trip Advisor) for a friend recently with the same questions (on a cruise w/ a day in PV). It's north of PV out past Bucerias. www.ranchobanderas.com

I would take a cab to Los cuatro Vientos Hotel high in the PV hills. Climb the steps to El Nido roof top bar for a fabulous view. Bar does not open to 5:00 but access all day for the view. Then go around the corner on foot turning left. You will soon arrive at Liz Taylor/Richard Burton's love "nests" with the connecting bridge between the 2 that spans the street. Then wander through Gringo Gulch with it's fabulous homes. Going down the hill visit the Island with several places for a cool drink or snack visiting the small but interesting museum.
Then wander through Old Town to Playa Los Muertos to Langistinos next to the dock. Great food, cleanest Banos on the beach & comfortable seating.
After take a cab back to the port.
A perfect first day visit to PV IMO>

Opps turn right after going around the corner.

Take the local bus, 6.5 pesos, to downtown, Map: http://vallartainfo.com/downtown.html, then as others suggested stroll along the Malecón, south, all the way to Los Muertos beach.

I would spend the day downtown at the beach and on the Malecon. Puerto Vallarta is a great, great walking town. Paradise Burger and Vitetta's (Italian) on the Malecon are very good and reasonably priced as is Fajita Republic in the old town, a block or two from the beach (I forget which street).

Fajita Republic is on B. Badillo