Re: Need a fantastic summer dessert!
OK, so you're outdoors in July for at least an hour. Presumably, you don't want dairy because that can grow bacteria, and you don't want anything that can melt in the heat. My first thought would be a bar of some sort: blondies, brownies, cobbler bars (shortbread bottom layer, thin layer of fruit filling, and then a streusel topping), or maybe seven-layer bars *if* you keep the chocolate chips in the middle so that they don't melt all over people's hands.
As for cakes, layer cakes can be risky because if your frosting or filling breaks down in the heat, the cake layers can slide around and turn into a big mess. So, I'd go with a sheet cake. Texas sheet cake is one option that comes to mind -- that's a chocolate cake with a sugary chocolate frosting that becomes a little brittle on the top, so it's almost like a frosted brownie (if you watch Cook's Country, that was one of the recipes they did a couple weeks ago). I've also seen peanut butter sheet cakes and sheet cakes topped with cinnamon sugar.