Documents needed to cross the border into Canada and back into the USA

Depends on your travel plan. The exact rules (air, land, sea) are spelled out here https://help.cbp.gov/app/answers/detail/a_id/619/~/visiting-the-u.s.---documents-required-for-canadian-citizens-%2F-residents-%2F

In addition to the "formal" rules, US immigration officers have a fair bit of "discretion" so there can be some significant variability there as well.

The easiest and obvious answer: get a passport. It's not that hard or expensive and makes travel quite a bit safer and easier.

To cross into Canada you'll need:

• for land, water, or air travel, a passport, or
• if crossing by land or water, a US passport card (unless you're also a US citizen, you won't be eligible for one), or
• if crossing by land or water, an enhanced driver's license (only a few states provide these, usually at extra cost) (only available to US citizens), or
• for air, water or land travel, a NEXUS trusted traveler card (Canpass will work as well, but this isn't valid for returning to the US)

If the border officer is satisfied with your claim of citizenship, it is possible you will be admitted without these documents, but you're taking a big chance as you may be turned back.

Here is Canada's web page on entry requirements for US citizens and permanent residents.

If you're flying, you'll need a valid passport or NEXUS card. For trips by land, as of April 26, 2022, US permanent residents are no longer able to enter Canada using only a green card. (See the edit history for a description of the requirements before that date.) Therefore, you will have to rely on the ability of Canadian citizens to prove their citizenship and identity using means other than a passport. The listed documents do not include the green card, however, nor US driver's licenses. I suspect that this is an oversight, but to be safe in case it isn't, you'll want to select from the documents on the list. Your proof of citizenship could be, for example, your birth certificate or a citizenship certificate, while proof of identity could be a credit card or a social insurance number card. The complete lists are below:

From Travel documents and identification requirements:

Acceptable documents to denote identity and citizenship

Upon arrival at a Canadian port of entry, travellers must satisfy a CBSA border services officer (BSO) that they meet the requirements for entry into Canada. For Canadian citizens, permanent residents and persons registered under the Indian Act, this can be done through questioning and through verifying documentation such as a:

• Canadian passport
• Canadian birth certificate
• Permanent residence card
• citizenship card
• Secure Certificate of Indian Status (SCIS) card or valid Certificate of Indian Status (CIS) card

If you do not have a passport, and are returning to Canada, the following documents can denote identity and citizenship:

• NEXUS card, held by a Canadian citizen, when entering Canada by air (when coming from the U.S.), land, or marine modes
• FAST card (Free and Secure Trade), issued to a Canadian citizen (when arriving by land or marine modes only)
• Canadian emergency travel document
• Canadian temporary passport
• Certificate of Canadian citizenship (issued from 1954 to present)
• Enhanced driver's license issued by a Canadian province or territory
• Enhanced identification/photo card issued by a Canadian province or territory

Other acceptable documents for establishing Canadian citizenship

The following documents may be used to establish Canadian citizenship. Upon presentation by travellers, the documents should be supported by other government issued photo identification:

• Certificate of Canadian citizenship (large form issued between January 1, 1947, to February 14, 1977)
• Certificate of retention (issued between January 1, 1947, and February 14, 1977)
• Certificate of naturalization (issued before January 1, 1947)
• Registration of birth abroad certificate (issued between January 1, 1947, and February 14, 1977, by Canadian citizenship authorities)
• Provincial or territorial birth certificate (individuals born in Canada

Other acceptable documents to support identity

The following documents may be used to establish your identity:

• CANPASS card (air, corporate air, private air, remote area border crossing), held by a Canadian citizen
• Commercial driver registration program card, held by a Canadian citizen
• Provincial and territorial driver's license
• Employment or student card with photo and signature
• Provincial health insurance card
• Provincial identity card
• Canadian Forces identification
• Police identification
• Firearms acquisition certificate
• Social insurance number card
• Credit card
• Vehicle insurance certificate
• Vehicle registration

Acceptable documents for establishing Registered Indian Status and identity

Every person registered under the Indian Act can enter and remain in Canada by right as per subsection A19(1) of the Immigration and Refugee Protection Act. The Secure Certificate of Indian Status (SCIS), more commonly referred to as the Status card, is an identity document issued by Indian and Northern Affairs Canada confirming that the cardholder is registered as a Status Indian under the Indian Act.

Note: The Certificate of Indian Status (CIS) was replaced by the SCIS in 2009. Existing versions of the CIS will remain valid until their renewal date.

As to reentering the US by land, a green card is sufficient:

What documents do I need to present to reenter the United States?

If seeking to enter the United States after temporary travel abroad, you will need to present a valid, unexpired “green card” (Form I-551, Permanent Resident Card). When arriving at a port of entry, a U.S. Customs and Border Protection Officer will review your permanent resident card and any other identity documents you present, such as a passport, foreign national I.D. card or U.S. Driver’s License, and determine if you can enter the United States. For information pertaining to entry into the United States, see U.S. Customs and Border Protection’s webpage.

The lack of a passport requirement for green card holders is found at 8 CFR 211.2(a)(2):

§ 211.2 Passports.

(a) A passport valid for the bearer's entry into a foreign country at least 60 days beyond the expiration date of his or her immigrant visa shall be presented by each immigrant except an immigrant who:

...

(2) Is entering under the provisions of § 211.1(a)(2) through (a)(7);

Where 8 CFR 211.1(a)(2) reads:

§ 211.1 Visas.

(a) General. Except as provided in paragraph (b)(1) of this section, each arriving alien applying for admission (or boarding the vessel or aircraft on which he or she arrives) into the United States for lawful permanent residence, or as a lawful permanent resident returning to an unrelinquished lawful permanent residence in the United States, shall present one of the following:

...

(2) A valid, unexpired Form I-551, Permanent Resident Card, if seeking readmission after a temporary absence of less than 1 year, ...