Went to Niagara Falls last summer. Check to make sure they didn't add any new rules but these are pretty standard.
Call your auto insurance agent and get a letter indicating Canadian coverage. No cost for me, just call a couple weeks ahead of time. Not sure you have to but read this recommendation somewhere and it was easy to get.
Have both of your passports. Also keep copies of your passports in your luggage or someplace in case they get lost.
Figure out your cell phone coverage. In my case I think calls were Roaming but not too expensive. It was the Data that was a potential killer. On our iPhone we went to Settings, General, Cellular and turned Cellular Data OFF. Then only used hotel WiFi to check e-mail, etc.
NO NO items are tobacco, liquor, weapons, fruit, radar detectors. Read the rules. This includes No Pepper Spray into Canada. Be careful about knives and consider declaring when asked about weapons. In my case when asked about weapons I said "I have a knife for camping" which I did along with our tent and camping gear. You can take a knife as a tool, but not a knife as a weapon. Be careful how easily any folders open.
Border crossing is pretty easy but the folks in the booths do take their jobs seriously. Hand them your passport and they will ask: Country of citizenship? Purpose of visit? How long? Any weapons, tobacco, liquor, fruit? May be additional questions but these are the standard ones. Pretty straight forward with all four of our border crossings that weekend. No inspection or any interaction beyond the four questions through our car window.
Keep receipts for your shopping. Read the rules for coming back to the US, like NO Cuban cigars, fruit, etc.
Enjoy your trip.
We take liquor and tobacco products every time we go. You are allowed a certain amount of each, just declare what you have when they ask.