We have always cooked for my son's birthday party which is always a pool party. We always have a fruit salad - you can do it in a watermelon if you want it to look festive. Homemade pita or tortilla chips (use pizza cutter to cut into wedges, toast in oven until crisp) with hummus and/or baba ganouj is easy and healthy. If you don't mind grilling, chicken and/or veggie skewers can be made in advance and grilled at party time. Little tea sandwiches are also fun finger food (cut the crusts off bread, spread with butter, flavored butter, cream cheese or other spread and top with thin slices of cucumber, radish or really whatever little topping you want - then use fancy cookie cutters to cut into shapes - or just cut each slice into 4 triangles).
LOTS of kids eat salad, fruit and a variety of other foods. You should not feel pressured into providing the 'usual suspects' found on all children's menus. They are there because they are cheap and easy for restaurants, not because they are nutritious foods children should learn to eat.
I don't believe there is any such thing as a homemade cake mix. You either bake a cake (really not a terribly complex thing), buy a mix or buy a cake from a bakery (or ice cream cake from an ice cream place).
Pinatas are fun but do require supervision. You do not want blindfolded children swinging sticks in crowded areas (especially indoors). Also - many pinatas nowadays have strings to pull, not so much the bashing until they break. It is lots of fun to watch little kids play either croquet or badminton. We have set up both on the lawn and just let people play as they wish.