Just about anything can be found cheaper in the states! But a lot can depend on which state you're in as well (different sales taxes apply.)
Along with melatonin, over the counter medications (generic brand) can be dirt cheap in the USA. This includes cold-and-flu tablets (just a few of dollars a pack), vitamins (300 multivitamins for a few dollars), pain relief (huge bottles for just a couple dollars), right through to things like ulcer medications (again just a couple of dollars a pack - compared to $20-$30 a pack in Australia).
As Russ says... clothing can be ultra cheap - especially clearance racks! Shoes are also about half the price... and that's across the range for things like Nike, dress shoes etc etc. If you know your size, and if your hotel will accept packages, you can hop on-line to Zappos and order all your shoes to be waiting for you (although most major department stores will be in the same price range, just the range of styles won't be a big).
Major brand cosmetics are also cheap (can be as little as 1/3 the cost of Australia).
If you go to some of the huge supermarkets, they have all sorts of interesting cooking utensils (cheap), and all of the 'As seen on TV' items - again all half to a third the price you'd find them in Australia.
(edit - just saw you will be in CA - the supermarkets aren't as cheap for medications there... but still can be great value compared to Australia).