I feel really *lucky* to have found Franky's place so last minute, if you can rent it, you are in for a treat. This location is great and the value is even better (there was an air conditioner which not all airbnbs have!). Franky provides beach chairs and an umbrella. Nearby there are plenty of places to rent snorkels. I recommend snorkeling, so many bright colored fish, it was a highlight of the trip! Don't worry too much about the weather, most of the days it rained for a few minutes and then was sunny 80-90% of the time. Of the ~7 days we were in Poipu, it said it would rain all of them and then only 2 of them it actually rained most of the day.
Poipu beach was really fun. During the day you might see some monk seals lounging in the sun. At night, you can see turtles coming out of the water to sleep (just don't get too close, a lot of people were getting too close, scaring the turtles and trying to touch them, it was sad). Great beach for families.
You can walk to the beach in a few minutes. I recommend renting a car if you plan on going out, walking to most restaurants, coffee, shopping is dangerous since there are no sidewalks (though people do it, we did for a few days too).
We walked the Heritage trail which had great views to Maha'ulepu beach which was pretty secluded, we saw a monk seal there and there are trees for shade. Just be careful there are no lifeguards. Franky has a suggestion for a running loop using the trail. If you're a runner, ask him about it.
For a few days, Poipu had strong currents and rip tides and you were only able to swim in a small part of it. I asked the lifeguards if there was a calmer beach and they recommended Anini beach but warned there are no lifeguards there. It was a really pretty beach, most of it was was for camping. We drove down right before there was a sign that prevented us going any further, also a great remote/secluded beach area right by that sign. We were told it was great for snorkeling (we didn't bring a snorkel, rent one before you drive there!) because it is the only barrier reef in Kauai (not sure if it's true, just what we were told). Also check the surf or ask the lifeguards because sometimes the currents are rough there too. It really all depends.
We drove up to Hanalei Bay for a day, but was rainy/cloudy/foggy. Preferred the south area better.
Restaurants: Brennecke's restaurant is expensive but the happy hour was fun. Keoki's was good for the chef's sampler (appetizer, meal, and dessert for about $26 per person). I also splurged and got the touristy drink in a pineapple and it was really good, super refreshing. My favorite though was Eating House 1849 Koloa where you can get a whole fried fish with some Japanese curry dipping sauce. Favorite meal of the trip. Puka Dogs was also a fun snack/stop.