Eager Beaver is on the money when he says that the usual questions are those in Uncle Kracker's post above. Numerous others have already posted the best advice be honest. Border agents are looking for smugglers, illegal immigrants, potential terrorists and other criminals, not for guys going to see sp's. If you look and act "squirrely" you're going to be questioned (or if you look out of place - going through a small border port because map blast said it was the shortest route, you live in Philadelphia you say you're going to Montreal but you're crossing the border in Maine [because you were seeing your cousin from California who was skiing]). Being honest, however, doesn't mean telling everything you're planning to do hour by hour. For instance:
Q: Where are you going? A: Montreal (or Ville de Québec or wherever).
To date I've never been asked what hotel.
Q What's the purpose of your visit? A Tourism, I'm going to: a) do the clubs, b) go to the Museums, c) go to the hockey game, and, and only and d) get together with some acquaintances.
The Border agents typically aren't concerned about whom you're seeing unless it seems likely you're seeing either a criminal or a terrorist.
Why are they concerned about you visiting a girlfriend or family? Because they don't want to get jammed up for violating the rules. If they have to ask Wong Li from the PRC if he or she is going to be visiting relatives in Canada and are concerned about them becoming an illegal, then they have to ask you as well. Even though they know that U.S. & Canadian Nationals have very little economic motivation to go illegal on the other side of the border and that it is relatively easy to get permission to work and or reside in the other country and that tons of Canadian and U.S. Nationals are married to each other.
Finaly, particularly if you're going to be a regular traveler. A NEXUS card is a nice privilege to have if you use one of the border crossings or airports offering the program. However, it is a privilege only extended to those who can pass the background checks and it requires an in-person interview (for most of those on this board probably in Montreal) with both U.S. and Canadian border protection agencies - but its a time saver and good for 5 years. That said, if the activities here are your only "vice" fine. But, if you have true vices, don't even think about NEXUS! There is a no tolerance policy for holders of NEXUS cards. You are expected to know the rules and if you violate them, both govenments have a policy of prosecution on all charges.
