Serious immigration restrictions...and he'd get away with it too. No student visas to muslim countries and a total overhaul of 1960s immigration laws in favor of a return to the 1920s laws.
He would likely bomb Afghanistan, but at the same time pull the plug of aid to Israel. He'd essentially punish the Taliban, but not seek to actually rebuild the country. He would then withdraw to fortress America and let Europe deal with the mess of Muslim terror in the 21st century.
Ironically, the US probably dodges serious follow-up terror attacks thus vindicating "the Buchanan Doctrine." It totally changes the landscape of the 21st century US, though this is not some sort of wank. Trade barriers and such probably lead to economic problems worldwide and the US essentially gives up global hegemony outright without a fight.