Along those same lines, we also line up various snacks so it is obvious that there are the same number of objects of each type of snack (put them in pairs). Then before the kids have time to count the goldfish, I put them into my fist and then see if they use the count of the different type of snack to tell me how many goldfish are in my hands. Sometimes we extend it when I have one of them slip an additional cracker into my closed fist or remove one and then ask the girls how many there are.
Most of the fun in this exercise involves pouring water into the cups and from one cup to another. I'm not sure if my girls really understand that different size cups of water can really hold the same amount of water when the water levels differ, but it's great for them to explore this concept, ask questions, and make predictions.
All these activities are so simple and require no preparation or special supplies, and the kids have a great time doing 'magic tricks' with math concepts.
UPDATE: I was so inspired by how much my kids enjoyed early math "games" that I created the La La Logic Critical Thinking Curriculum for 3-6 year olds, which includes online brain challenge games, printable worksheets, enrichment activities and more!