Three days is enough to see the three faces of Kauaʻi: the green north shore, the sunny south, and the canyon country out west. The trick is to stop trying to do all of it and instead pick one region per day, because the island has no cross-island road and every drive is an out-and-back. Here is a sane plan that keeps you out of the car and in the water, with the ocean conditions checked before you go.
Kauaʻi has two highways that both start in Lihue and dead-end. Highway 56 runs north toward Kapaʻa, Princeville, and Haʻena. Highway 50 runs south and west toward Poʻipu, Waimea, and Kokeʻe. They do not connect. The Na Pali Coast sits between them and no road has ever been built through it. So group your days by direction, not by a mental checklist. Roughly: Lihue to Hanalei on the north is about 45 minutes and 30 miles, and Lihue to Waimea Canyon is about an hour.
Base your first day around Poʻipu on the south shore. It tends to be the sunniest, driest part of the island, and in winter it is usually the calmer coast while the north shore is big. Poʻipu has lifeguarded beaches and some of the better snorkeling on the island. No beach here is ever a guaranteed safe swim, so check that day's conditions on today's conditions, get in only where there is a lifeguard when you can, and follow the posted signs.
Drive north to Hanalei. In summer (roughly May to September) the north shore calms down and reefs like Anini and Tunnels draw snorkelers, though both are largely unlifeguarded, conditions-dependent, and only beginner-friendly in the protected inner reef on low-surf days. In winter the same water turns into world-class surf that is dangerous to enter. To reach Haʻena State Park at the very end of the road, where the Kalalau Trail starts, non-residents must reserve in advance: a $5 per person entry reservation plus either a $10 per vehicle parking reservation or the North Shore Shuttle, and these sell out fast. A day-use reservation lets you hike to Hanakapiʻai Beach (4 miles round trip) and on to Hanakapiʻai Falls (8 miles round trip) with no separate hiking permit; continuing past Hanakapiʻai Valley toward Kalalau needs a Nā Pali Coast camping permit, even for day hikers. One hard rule: do not swim or wade at Hanakapiʻai Beach. It has no reef and no lifeguard, and its rip currents and shore break have killed many people even on calm-looking days. Treat it as a viewpoint, not a swim stop.
On your last day, head west to Waimea Canyon and Kokeʻe (about an hour from Lihue), which is the easiest way to see the dramatic interior from the ground. The other strong option for a short trip is a tour that does the seeing for you: a Na Pali boat tour in summer when the water cooperates, or a helicopter tour any time of year, since more than 70% of the island cannot be reached by car. Mornings tend to be clearer for both.
Pick one region per day and stay there. Do your ocean activities in the morning when wind and surf are usually lower, and treat the afternoon for drives, food, and views. Pack reef-safe mineral sunscreen before you arrive, because Hawaiʻi law bans the sale of sunscreens with oxybenzone and octinoxate. And check the daily beach conditions the morning you go, not the night before.