From Britannia ride neighbourhood streets to the Champlain Bridge, then north through Moore Farm and Parc Champlain and ride the bike path beside 148 to Aylmer. Continue west along the Ottawa River until the pavement ends. Back to Aylmer for lunch. Return to the Champlain Bridge through neighbourhood and bike path, then follow the bike path along the Ottawa River all the way back to Britannia.
Note that Garmin gets very confused in the labyrinth in Parc Champlain, but smartphones are fine with it.