Oh man congrats on the a6400!! You are gonna have a total blast in Hawaii! Honestly, the 18-135mm lens is the absolute king for travel especially on a budget. I literally refuse to travel without my Sony E 18-135mm f/3.5-5.6 OSS because it just works every single time and I trust it completely. I remember my first big trip to Kauai... I was so nervous about getting the perfect shots that I brought three different lenses. Big mistake. I was trying to switch to a long zoom on a boat tour near the cliffs and almost dropped my camera right into the salt spray because the boat rocked! It was terrifying and I probably got salt air and dust inside the body too... never again. Since then, I just use the one zoom so I never have to open the camera up in the elements. The image stabilization on that Sony lens is fantastic for keeping things sharp when youre on the move or even if your hands are a bit shaky. Plus it fits right into your $600 budget if you find a good deal or buy it used! Quick tip tho: definitely grab a protective filter for the front glass. The volcanic sand in Hawaii is super abrasive and can ruin your lens if you are not careful. You wont believe how amazing the photos look!! Its basically the only lens you will need for those sunset beach shots and the hiking trails. Have a fantastic time!!