For Americans wanting to get into Canada quickly, the Derby Line/Stanstead crossing is ideal. The wait at the border is much less than that at the major border station down by the highway. Having said that, don't be fooled by the absence of physical barriers running through the border. In fact, one street has some houses on the American side (Derby Line, VT) and the rest on the Canadian side (Stanstead). Don't cross Canusa street to visit your friend who lives right across you; you need to to report to border agents (with valid passports and visas) before having a meal with them. Crossing the street without proper inspection into either side can put you in serious trouble with the authorities of the country you are trying to enter.
The lone exception is visiting the bi-national Haskell Free library and opera house (purposely built to straddle the border). You need to enter through the door on the American side (the back door is locked anyway). Once you exit the building, you must immediately return to the country you entered from.