One good thing about the bad economy: There are plenty of empty storefronts in malls across the country, and the owners are more than happy to rent them to short-term tenants during peak seasons. And, this year, you can add Toys R Us to the usual gang of Halloween and Christmas stores that pop up suddenly in your local Simon, and disappear in the night a few weeks later. Jeffrey and his posse are expected to open 80 temporary stores next month, along with new toy sections in 260 Babies R Us shops.