Crossing the border in a car is pretty easy. They're not going to make you even get out of the car in most cases. If you're going into Canada, remember there's a fair chance you'll get someone that speaks French as a first language and barely speaks English (despite the fact they're supposed to be fluent at the border. I went round and round with someone like that until I figured out that Green Paper meant the registration for my car). Usually, they'll ask you general questions about where you're from, where you're headed, for how long, whether you're bringing any firearms, alcohol, or tobacco into the country, and a few other things. Keep calm, answer succinctly, and you'll be fine. The Canadian guards generally are much much friendlier than are the U.S. guards, as a heads-up for your return trip. They might ask you to cut off the engine and open the trunk, at which point they might paw around your bags a bit. Only if you seem really nervous or are otherwise acting odd, or they find something they don't like, will they order you for additional screening.
The worst that happens is that they reject you entry into Canada (unless you're actively trying to break the law by smuggling drugs or something) and send you back. If that happens, explain what happened to the U.S. guard and they shouldn't give you too many bumps.
Back in 2008 or so, I went to a big diaper fest in Vermont by way of Canada. I had a case of Bambinos, my stroller, bags full of ABDL clothes and gear, and my steel spiked collar. I was going to drive half the trip, sleep in my car at a rest stop, then finish the drive the next day. I hit the border in Windsor at about 7:00. Apparently my nerves over the trip and crossing the border and everything else made them suspicious, so off to secondary screening I went.
They pawed around a bit, until they found the pacis in my bag. That made them think drugs, and off they went. They disassembled my car. Three hours later, the contents of my car, two of the four chairs, and some of the interior panels were spread across two parking spots in the secondary screening area. They considered the spikes on my collar to be weapons, and they found a set of lockpicks I had lost months before (they were in the overhead compartment all the way to the back behind a sunglass case, apparently).
Of course, I felt like I was going to die of embarrassment! They must have picked up on that, and at the end of it all, one of the guards said to me, "You know, we don't care what you do in your private time. We're federal agents at the busiest border crossing in Canada, and we have way too much stuff to do to worry about your private life. As long as you're straight up with us, we don't care."
And that was it. I probably gave them a story for the ages, but they actually helped me put my car back together and were courteous and very professional. They did refuse me entry, as apparently lockpicks are illegal to possess in Canada and they thought the collar could be used as a weapon. Before I could leave, though, I had to go to the immigration office, where they grilled me about my job, whether I was trying to get into Canada to work illegally, my finances, how much money I had in my bank accounts (since I had $8 in cash on me), and a bunch of other crap. After I finally convinced them I was just passing through, not trying to start a business in Canada without a work permit, I went home, dumped the oddball stuff off, got some sleep, and went back the next day. After a brief stint in secondary screening (they didn't search the car, just asked me a bunch of immigration stuff), I was finally on my way.
I've crossed the border numerous times since then without much problem. About the closest I got to a problem was the episode last year with the French guy that kept harassing me about my green paper. It turned out he didn't like the temporary tag on my new car and didn't speak enough English to competently ask me for the registration. After going round and round with him and figuring out he was on about the car, I handed him the whole stack of paperwork from the dealership, my registration, and proof of insurance, and he finally figured it out and left me alone.
