As a local to Oahu, yeah there is traffic. Not cali on dem freeways bad, but still got a lot of traffic in certain areas including waikiki as you would expect. Traveling to North Shore and from North Shore direction at certain times might have you hit some bad traffic due to rush hour.
The beaches dont get busy like a number of the European beaches, but there be people with usually parking being the biggest issue. For restaurants it all depends where you go. There are a number of places in Waikiki itself that are not that long of a wait if there is one.
Kailua has gotten even more popular over the years with a lot more tourist activity as well. The beaches are not so crowded as much as parking can be a big problem.
To OP, have fun. Not sure if you are traveling alone but that is one thing that would freak me out with traveling and havent done yet.