The notice periods printed on these listings are the most useful timing information here, and almost nobody reads them until it is too late.
Three hours, on the panoramic flight. That is the minimum the booking will accept, which makes a same-day ten-to-forty-five-minute flight possible if you decide over breakfast. It does not make it likely in a busy week. The listing carries 19 reviews at 4.9 and a private option, both of which point at small capacity, and small capacity is what sells out first.
Ten hours, on several of the ground tours. The vineyard and cider day at $126, the four-stop Canadian food walk at $97, the private Christmas walk at $349 and the private tour of the corners most visitors miss at $581 each ask to be booked at least ten hours ahead. In practice that means the evening before, not the morning of.
Fifteen minutes, on the cheapest flight over the capital. The length is the reason this booking is easy to place in a day: it is shorter than the drive to most things, so it slots in before a two-hour walk or after a three-hour market tasting without either of them moving.
Two hours, on most of the walking bookings. Eight of the eleven ground listings run two hours or less, which is the other half of why an Ottawa helicopter tour is easy to place: the things you would pair it with are short as well. A flight and one walk is a half day with room left in it.
Put those numbers together and the plan writes itself. Book the flight for a morning early in your stay, book the ground tour you want the night before, and keep one afternoon with nothing in it. That last part is what turns a cancelled Ottawa helicopter tour into an inconvenience rather than a missed trip.