Being part of the US, the rules of the road are the same in Kauai as they are on the mainland. However, the culture in Hawaii is generally much more laid back. You'll rarely hear anyone beep their horn, in fact it's considered rude to do so. There are also tons of wild chickens that will wander onto roads, along with the endangered Nēnē (Hawaiian Goose). These geese are protected, and you may see road signs notifying you of areas where they tend to cross.
One great thing about the beaches in the state of Hawaii is that they are all state land and open to the public. That being said, there are some beaches on every island that can see big waves and dangerous rip currents, and will usually have warning signs posted. Some of the most popular beaches for swimming on Kauai are Poipu, Anini, Tunnels, Hanalei Bay, Ke’e, Lydgate, and Kalapaki located right at the wedding resort. The majority of beaches on the islands have restrooms, changing rooms, ample parking, and rinse stations.
Not ironically, some of the top beaches for snorkeling are also the best beaches for swimming as the waters are calmer and better for visibility. Poipu, Anini, Tunnels, Ke’e, and Lydgate have some of the best coral formations and underwater views. You may even luck out and see some sea turtles along with all the beautiful fish. That being said, sea turtles (honu) are protected and it is illegal to touch them or get too close if they are on the beach. If you don't have your own snorkel gear, some resorts rent them, or you can go to dive shops like Boss Frogs or Snorkel Bob's for deals on weekly rentals.