Bar hopping in Gulfport can be interesting... they have a really great italian restaurant on the main drag there. Nothing on the water, but there are a couple bars/restaurants are across the street from the water. It reminds me of an extremely miniature Key West.

St. Pete's laid back and has a beach vacation sort of vibe, but it's a 15-20 minute drive to the beach; beach themed restaurants isn't really what we do. Anyway, here's my recommendation for your Saturday:

1. Market. The Saturday morning farmer's market in the parking lot of Progress Energy Field (1st St S @ 1st Ave S) is pretty cool. It's not there during summer, so this is for a later trip, but show up late morning and split an empanada (trust me). Summertime, there's a scaled-back market in Williams Park, but it's not worth prioritizing.
2. Lunch. If you need a beach-theme, try Fresco's (2nd Ave at N Shore). But go to Bella Brava a block away instead. Grab an ice cream at Cassis bakery if it's hot out.
3. Active. Take a stand-up-paddleboard lesson at the beach at North Shore Park (parking lot is at 12th Ave NE). I think you can arrange through Reno's surf shop on 4th Street. Be sure to wear polarized sunglasses so you can see all the (small) sharks and (big) stingrays under you in the shallows...
4. Art. There's some really creepy art, and a really cool building, at the Dali Museum. The cafe out back is a nice spot. Also worth checking out the Chihuly blown glass collection at the Morean Arts Center.
5. Dinner. Start by splitting a dozen at the Central Ave Oyster Bar. But don't have mains there. Walk over to the St. Pete Brasserie for duck confit (my favorite meal in St. Pete so far...). By the way, basically all of St. Pete is flip flop casual, even at dinner; don't worry about the post-paddleboarding look.
6. Cocktails. If you're having just one, try the rooftop deck at the Birchwood Hotel (excellent view) or go old school on the porch at the Vinoy. Alternatively, get a hotel and party on Central between 2nd and 3rd St.

Walking along Beach drive is nice. Cute stores and nice restaurants. If you're in the mood for gelato, there's an excellent place called Paciugo on that strip. And I do think that farmers market BBI mentioned at Williams Park is nice to drop by. They have really awesome knishes for lunch