(as requested) A luau is always a great "go-to" theme for summer parties!
I love this table because it shows how a few well placed, simple pieces of palm and netting really help say "luau" without over doing it! You don't have to put a tiki and a lei on everything! Source
Create these fun topiaries with paper parasols to add a fun details! Source and tutorial
You have to have coconuts! How about doing a signature drink served in half coconuts! Source and recipe You could pour candle wax in and make them into candles too!
Another simple touch - have a basket full of flip flops for your guests! Another basket with towels for wet activities and another basket of light blankets for the late evening would not only look great, but serve your guests well! Source
You know I am going to find a way to incorporate cupcakes! lol These hibiscus flower topped cupcakes are beautiful! Definitely time consuming, but try doing just a handful among plain ones - same wow factor and WAY less time! Source and tutorial