Five days is the sweet spot for Kauaʻi. It is enough to settle into one region, take a tour, and still have a buffer day for the weather, which you will need at some point. This plan pairs each day with the part of the island that is usually calmest for that season, so you spend more time in the water and less time backtracking.
Kauaʻi is built around two non-connecting highways out of Lihue: Highway 56 north to Princeville and Haʻena, Highway 50 south and west to Poʻipu, Waimea, and Kokeʻe. Lihue to Hanalei is about 45 minutes; Princeville to Poʻipu is roughly an hour and a half across the whole island. With five days you can base in one area and take day trips, rather than packing and moving. A north-shore or south-shore base both work; south tends to be drier and sunnier.
Start easy near Poʻipu. It is usually the sunniest coast and, in winter, generally the calmer one, with lifeguarded beaches and good family snorkeling. Use day one to get groceries, find your beach, and adjust to the time change. No beach is a guaranteed safe swim, so check that day's conditions on today's conditions, favor lifeguarded beaches, and remember even calm-looking water can hold currents.
Drive west (about an hour from Lihue) to Waimea Canyon, the 'Grand Canyon of the Pacific,' and on up to Kokeʻe for the lookouts. This is the dry side, so it is a good rain-buffer day. Bring layers, since Kokeʻe sits at elevation and is cooler than the coast. This is a viewing-and-hiking day, not an ocean day.
Because more than 70% of Kauaʻi cannot be reached by car, a tour is how you see the rest. In summer, a Na Pali Coast boat tour is the classic choice when the north-shore water is calm. A helicopter tour works year-round and reaches the interior, including the Waiʻaleʻale crater area, one of the wettest places on earth at about 450 inches of rain a year. Book a morning slot for the best odds on clear weather, and build in flexibility because tours do get weathered out.
Spend a full day up north. In summer the north shore calms down and reefs like Anini and Tunnels draw snorkelers, though they are largely unlifeguarded and only beginner-friendly in the protected inner reef on low-surf days; in winter the same beaches are for watching surf, not entering it. To reach Haʻena State Park at the road's end, non-residents must reserve ahead: a $5 per person entry reservation plus either $10 per vehicle parking or the North Shore Shuttle. A day-use reservation covers the hike to Hanakapiʻai Beach (4 miles round trip) and Hanakapiʻai Falls (8 miles round trip) with no separate hiking permit; going past Hanakapiʻai Valley toward Kalalau needs a camping permit. Do not swim or wade at Hanakapiʻai Beach: it has no reef, no lifeguard, and rip currents and shore break that have killed many people even on calm days. It is a viewpoint, not a swim stop.
Use the last day to redo whichever beach or town you liked best, hit a farmers market, and ease toward your flight. Keep ocean activities in the morning when conditions are usually gentlest, and leave the afternoon loose. If a tour got canceled earlier in the week, this is your make-up day.