Obama on Tuesday will return to the site of his nomination to sign the most important bill of his presidency: the $787 billion economic stimulus measure. The president will make his legislative victory official at the Denver Museum of Nature & Science. In his weekly address yesterday, he said the bill,passed by lawmakers in Washington on Friday, marked a "major milestone on our road to recovery," but warned of tough times ahead. "This historic step won't be the end of what we do to turn our economy around, but rather the beginning," he said. "The problems that led us into this crisis are deep and widespread, and our response must be equal to the task."