When President Trump signed his marquee domestic policy bill this month, the central tenets of his domestic agenda — slashing taxes and cutting deeply into social safety net programs — became law.

So did a behemoth number of other narrowly targeted, little-noticed measures — some that might have seemed random — that hitched a ride on the legislation, several tucked in at the last minute.

There are tax breaks for the rum industry in Puerto Rico and the U.S. Virgin Islands, Alaska whalers and metallurgical coal; $300 million to help pay for protecting Mar-a-Lago and Mr. Trump’s other homes; lease increases for two major Washington-area airports; $150 million to help celebrate the 250th anniversary of the founding of the United States; and $10 billion for space exploration missions to the moon.

Republicans even crammed in the major elements of a stalled farm bill, with provisions that benefit big and wealthy farmers, especially those in the South.

Clocking in at nearly 900 pages, the size of the bill ballooned in part because of these unrelated measures, turning it into the type of legislation often derided on Capitol Hill as a “Christmas tree,” because it becomes adorned with ornaments, often parochial add-ons, meant to win over enough lawmakers to pass.