Banff in May or September is hard to beat. You get pale turquoise lakes and crisp mountain air without the wall-to-wall crowds that pack the trails around Lake Louise in July.
A solid budget for two lands right around $3,200 for flights, stays, and park passes. Spreading that total cost over time gives you room to breathe before you ever step onto the plane.
A 0% intro rate is a deadline, not free money. Make sure you pay at least $177.78 every month so your balance hits zero before the 18 months run out, or standard interest rates will apply to whatever is left.