Between Quebec City and Levis, the St. Lawrence River is only about 1 km wide, which makes for a short, pleasant ride (about 10 minutes) aboard one of the two ferries that make the run, the Lomer-Gouin and the Alphonse-Desjardins. During summer, there's a ferry that departs every 30 minutes during the day, and every 60 minutes in the evening (check the Website for schedules), and tickets are only $2 ($4 for a round-way trip). On most days you'll see lots of people standing on the deck, taking pictures of Vieux Quebec and the Chateau Frontenac. A really fun way to begin your day-trip to Levis!
If you're traveling with young children or elderly people, or if you simply can't bear the idea of climbing 135+ steps, you can buy a shuttle ticket ($1) at the ferry terminal. The shuttle will take you all the way to the top of the cliff, and it's also the best way to get to National Historic Site Fort-Levis if you're on foot. Shuttles run from late June to early September, from Thursday to Sunday, between 11 am and 6 pm.
I suggest you take the boat from Québec City to Levis, the cost is only 2 $ and the trip is very short but you have a very nice view with Chateau Frontenac and Vieux-Quebec in one side and Vieux-Lévis in the other.