Tom Collins is a favorite, 2 parts gin, a good squeeze of lemon juice, pinch of sugar, splash of club soda, top with a wheel of lemon.
I've always found gin/vodka tonics to be very refreshing too.
Another favorite summertime drink horchata and spiced rum. Horchata is a sort of cinnamon milk type mexican drink, you can buy it at mexican stores. Pour some over some ice and top it with captain morgan.