I brought a baler and a bale wagon into Washington from B.C. last year. I got into Canada with just a driver's license and got back out with just a driver's license. All I needed for the equipment was a receipt and they charged me a small fee for the equipment - just a few dollars. When I came back later with the second piece of equipment, I told them it was from the same purchase and they waived the small fee.
I discussed the lack of a passport with one of the US agents before going into Canada. He said if you are a US citizen you will always be able to reenter. He said the passport speeds entry up and without it, they may take you inside for some questioning. They never questioned me.