You can cross the border with a passport or a passport card (but not by air with a card). The days of using my commercial radio license to prove my citizenship are long gone.

If you transport more than $10,000 in cash or negotiable securities (no signature travelers checks, bearer bonds etc.) without informing them at the border the entire amount can be confiscated.

I often carry checks made out to me of considerably more than that. Although I don't have to, I declare them, but they just wave me through as the bank transaction is available to the IRS.