Semi Formal/Formal
If you are only visiting Kauai then it may not be needed. The hotel/venue is only a 30 min drive from the airport so an Uber/Lyft could suffice. However, if you plan on visiting some of the other islands like Oahu then a rental car may be needed.
Lihue Airport. The identifier is LIH. However, depending on prices, it may be cheaper to work 1-way flights into Honolulu HNL and travel between islands from HNL to LIH.