It's on the east side of the island, about midway between Princeville to the north and Poipu to the south. It has like 4 or 5 very large and amazing pools. I think the Princeville area is the most beautiful if you can afford it. Hee Fats shave ice is the best I've found in Kauai.

You can hike a potion of the Na Pali Coast coast hike and then cut up to see Hanakāpīʻai Falls and swim in the pool beneath it. It's a challenging hike (my 12 year old and 10 year daughters did it, but it was difficult), but maybe the most beautiful in all of Hawaii. Here's a pro tip: you have to make reservations to enter the Na Pali hiking area well in advance, but you won't know at reservation time what the weather will be like when you get there. You don't need a reservation if you can beat the gate guard to the entrance. I'd say get there at 6:20 am before they show up at 6:30. You'll need to be dropped off if going this route. It's an 8 hour ordeal, bring lots of food and drink.

My kids really like Polehale Beach (it's a drive), but very remote and beautiful. Secret Beach is a favorite as well. On the far left side of the beach (facing the ocean), there are some natural pools to swim in. If you need affordable food that is still authentic and decent, Foodland grocery stores make fantastic prepared foods including really good Poke.