Has anyone done it recently without a Canadian passport, but are citizens? I heard its a bit of a mare these days.

Tks!

Yep, you need a passport or an enhanced driver's licence.

Tks BD.

Not the same but my question is:

If I go over the border crossing at Peace Arch, is my Green I - 94 issued by the U.S. valid for future crossings within the expiry date or do I have to go thru the same procedure every time I cross ?

Yes.

Or a NEXUS card, which is the cheapest option plus it can be used for air travel (with any passport).

The I-94 is valid until the date stamped on it, so you can cross back and forth without getting a new one, but unless you cross frequently (i.e. more than once every 90 days) you basically have to get a new one each time you cross.

However, crossing by land too frequently on an I-94W (green, Visa Waiver form) runs the risk of having some Yankee jobsworth deciding that you cannot participate in the Visa Waiver scheme on the basis that you no longer appear to be a permanent resident of the UK - as happened to me here on the southern border.

Or a Trusted Traveller Card (e.g. NEXUS, SENTRI, FAST).

There are also different rules for children aged 15 and younger as well as those aged 16-18 who are travelling with an organized group (e.g. school group, religious group, sports team, etc.) They can also use a birth certificate or a citizenship certificate.