SPI is very close to my hometown. I grew up at the beach every weekend. Like everything, it has grown and changed so much since I was a kid.
Weather wise, it will be very mild. When we go home for Christmas, we always make a drive or two to the beach and we wear shorts and a light windbreaker.
January is off season so the island will be very quiet. A great time to visit. Check out the Sea Turtle Sanctuary and the Birding Center. Book one of the boat tours just for fun or fishing. Rent a golf cart or dune buggy to tool around the island in the open air.
Restaurants: Dirty Al’s is a dive but the most popular seafood restaurant among locals. The owners of Dirty Al’s also own Josephine’s and Daddy’s, you can’t go wrong with any of those three. Blackbeard’s has been around forever. It used to be one of the only sit-down restaurants on the island when I was a teen. It’s still very popular for bar and grill type food.
Bars with a view: Wanna Wanna bar and grill is a tiny beachfront bar attached to a beach motel that is very popular among the locals. Great drinks and pub grub. Louie’s Backyard has been around since the 70’s. It’s a restaurant and bar with decks overlooking the Laguna Madre Bay. I’d go there for drinks and view and not the food.
For super authentic Mexican food you’ll want to eat across the causeway in Port Isabel or drive into Brownsville, Los Fresnos or Harlingen. If you want tips about any of those towns, let me know.