A.S.
You could have coloring and sticker books, face painting, water balloon tosses, a baby pool (or even just a sprinkler to run through if it's warm), bucket toss, "archaeology dig" (use a sandbox or rock area to bury little plastic toys and give each of the kids a bucket and shovel to dig for the "prizes" - can take a good hour and it's great!), and I agree that a pinata can entertain for a good long while, too, though maybe not the 5 month old so much... :) Bubbles are always good for a wide age range, too.