Yes! There are many places to see on the island and most of them only a short drive away! One of the hotels we recommended did have a 24hr car rental deal, something to look into if you only want a car for one day or so.
We are staying on the Coconut Coast (Kapa'a and Wailua) because it is centrally located to both sides of the island. The North Shore (Princeville and Hanelei) is beautiful but this time of year the weather can be much rainier and the beaches are usually too rough for swimming. Also keep in mind we are getting married on the south side of the island and that would be about a 3 hour drive at least to get to! The south side will have the best beaches and calmer waters, Poipu is a resort area so tends to be a bit more expensive.
Whatever you want! Hawaiian casual is laid back, summer/beach dresses for girls....shorts or pants and shirts are fine
If we are fortunate enough to get to the Napali Coast expect some amazing scenery! It is also whale season so we can be on the look out for those! The seas may be rough this time of year so if you get seasick or don't know if you get seasick plan on bringing something with you to help. Kids under 5 are not recommended to come on the boat due to the long 4 hour trip being stuck on a boat! Pregnant women in the 3rd trimester are not Allowed on the boat! Bring your sea legs and leave your shoes (not allowed on boat) and it will be a Great Time!