A valid passport is required for entry into Canada, and it must be valid for at least 6 months beyond the wedding date. Please check your passport and renew it as soon as possible if needed - processing times can be lengthy, especially around holiday periods.
The closest and most convenient airport to Niagara-on-the-Lake is Buffalo Niagara International Airport (BUF). Both Southwest and Delta offer direct flights from Tampa.
Since this is a holiday weekend, please be sure to book your rental car well in advance. All major rental car companies operate out of BUF.