Legislate

Congress can require presidents to disclose their tax returns. Congress also can require presidents to divest of their active businesses, toughen anti-nepotism laws, prevent self-dealing (profiting from the presidency while in office) and limit taxpayer reimbursement for personal travel (e.g., back and forth to Trump's properties). If Trump chooses to maintain financial secrecy and hold onto his businesses rather than remain in the presidency, that's his choice.