I'm speaking from experience as a former airline pilot and recreational (small aircraft) pilot who uses in the upper Midwest. You can't just fly over the border into Canada and land on a "neighborhood" runway in a private or small chartered plane. As soon as you are across the border you will be tracked on radar and you will likely be detained on landing as you now fit the profile of a drug smuggler. Depending on what the national security situations is - on a bad day you could get a friendly escort of F-16s, escorted to a secure airport and taken into federal custody.
On a good day, entering the country by crossing into its airspace without filing a valid flight plan would constitute a crime of illegal entry. Landing in an out of the way location without a valid customs zone could be construed as trying to avoid registering your presence to the government, declaring imported goods, paying taxes, or a more nefarious purpose.
The requirements (outside of the usual requirements as far as pilot license and aircraft registration documents, Canadian liability insurance, etc) are:
The pilot and ALL passengers must have passports.
A flight plan is required. eAPIS is the electronic system to file notification of an intended flight into Canada. You must submit an eAPIS notification at least one hour prior to departure. (requires an eAPIS account by the aircraft operator, which if they don't have already, can take up to a week to get).
All cross border flights require an annual Department of Homeland Security (DHS) sticker on the aircraft. The sticker should be ordered in advance since it may take several weeks to arrive.
When going into Canada, you must call CANPASS (1-888 CANPASS) and again, you MUST be on a Flight Plan (either IFR or an activated VFR). The first landing MUST be at an Airport of Entry.
This is just an overview and not all-inclusive. The
Customs and Border Patrol website has the information.