Depending on the mood of the border guard, you might want to not bring any fresh fruit or meats. Just plan on buying that stuff when you get there. Just have pre-packaged food on board. I have not quite figured out the deal with fruit and meat, so we just don't have it on board when we cross.
Don't joke with the border guard. Again, this can depend on the guard. Have your passport ready - they are usually not interested in your driver's license. Also have your vehicle registration and insurance ready. They might ask what the license plate numbers are - don't panic if you don't remember, a lot of the questions are really just to see your reactions and see if they get odd responses. You also might get a couple questions that are just different versions of earlier questions - they are just seeing if your answers match (I was first asked what day were we returning... last question was "how many days are you staying?"...).
Don't panic if your camper gets pulled over to be inspected. Usually it is just a quick walk through, seems random most times as far as I can tell.
We've been to Niagara Falls twice the last few years. It is a BUSY border crossing in that area, so my experience comes from that. The border guards are just looking for things out of the ordinary and suspicious. One day the guard might be joking around a bit, the next day they might be all serious business.
Have fun, we've enjoyed our visits to Canada!