Flying to the USA from Canada and back is not that complicated. You have to file a flight plan (VFR or IFR) and be in contact with ATC before crossing the border. Your first point of landing in the US must be one with customs service, and you must notify them of your ETA before taking off from Canada. From CYHU, the easiest place to clear customs is KBTV (Burlington, Vermont). From there you can fly onwards to your US destination. You can also clear customs at your final US destination, but bear in mind that if you have to divert enroute because of weather, you will have a ton of problems if you land at a non-customs airport or one where they are not expecting you.
US customs charges 25$/year for a customs user fee which is shown by a decal applied to the aircraft. If you're renting, check if the aircraft already has a current customs decal, otherwise bring 25$ cash in US currency with you. I've had a friendly US customs agent waive the decal fee when they saw I was renting the aircraft.
On your way back to Canada, you must call Canadian customs with your ETA before take-off. You must land at an authorised airport on your way in (CYHU is one) and wait for Customs agent to show up on the ramp before leaving the aircraft. If they are not there, you must call them again. Sometimes they will just ask questions on the phone and give you a clearance number.
I found a very detailed guide to these procedures linked from the Rockliffe Flying Club (Ottawa) web site here:
http://ca.geocities.com/[email protected]/Canus.pdf
If you are flying to the US from Montreal, Boston is about 1.5 to 2 hours and the approach into Logan airport is spectacular (over the water on the bay, near downtown). As I recall, landing fees are not too expensive if you don't land at peak time. About 1 more hour's flight is Cape Cod, with many resort towns and beaches, including Provincetown (KPVC) where the airport is next to the public beach. You can also fly to New York City (about 3 hours in a C172) and land at Kennedy airport (KJFK) where they also have reasonable off-peak landing fees. There is a useful site where you can find info about US airports, where to park, costs, etc. at
www.airnav.com