IoS has ben my go-to kilt for many years. Mine is from the Lochcarron mill which if I remember right, has more subdued colors in IoS than other mills. IMO it is one of the most versatile tartans when it comes to accessorizing. As OC Richard's pic above show, you can do just about anything with it. Dress it up with black leather, shiny metal and a fine wool vested Argyll, or dress it down with brown leather, brass or antiqued metal and an earth-toned tweed jacket (which is my preference, although I am putting together a more formal set up).

As for what type of shirt, I've always been rather casual above the waist, especially alternating between NorCal and SoCal, where it can be rather hot when a lot of our events are held. The Pleasonton games (triple digits sometimes) are t-shirts for sure, but the Seaside games in October are sweater weather. In Hawaii, you're probably looking at lighter fare. I think something like a lightweight t-shirt in any shade of gray will work just fine.