this is one suggestion for a walking tour starting at the bus/train station https://goo.gl/maps/21GMdcBdZey but instead of walking from Lower Town to Upper Town via the staircase you can take the funicular (which google walking mode will not allow) the funicular is a good investment when trying to see as much as possible.
You can either walk in to the Citadelle and take the tour (1 hr +waiting for when it starts) or just walk around to get to the Plains of Abraham where you find a stair that goes down to places des Gouverneurs near Chateau Frontenac.
The Chateau Frontenac is the landmark to get you back on track if you wander on some side streets.
a streetview of the access to the stair after the Citadelle https://goo.gl/maps/mMroSFQLp472
the funicular to Upper Town (Haute Viller) on the left - the stair straight ahead https://goo.gl/maps/uDpLdjVXu482
Almost everybody seems to miss the point that historical Old Québec is not only the walled city in Upper Town... To know a bit more about the layout of the area visit these Top Questions on the upper right side of this page (not visible on smart phone, you need a computer): What is the difference between Upper and Lower Town? I am confused about the layout of the city, how does it work?
So I really like the ittinerary that cbaarch suggested. In fact on my last visit I did half of it, meaning that I stopped at Saint-Louis Gate and then just wandered around in the intramuros part. This ittinerary will bring you to Promenade des Gouverneurs that has a lot of steps but from Monument Abraham Martin going back to Terrasse Dufferin you will have more going down than up. Oh and I also took the funicular to get in Upper Town. I would skip the tour of the Citadel and instead just wander around a bit more in the Lower part before taking the funicular or the Breakneck (Champlain) Stairs to Upper Town and then wander around this part more once you have circled Cap Diamant by Promenade des Gouverneurs. You will have a lot of options for cafés and pubs all around to relax a bit and you can take a cab back from Place d'Armes / Château Frontenac or from Place d'Youville to the train station, it is a short and inexpensive ride downhill.
I would, if time permit take an earlier bus and/or leave later (if you have modifiable ticket, if you bought the 25$ ticket then forgot it)Edited: 24 May 2018, 04:26